fix(#4636,#4653): close the symlink hole, revert a wrong collapse, fix six review findings

The RED checkpoint and two orthogonal reviews found eight defects. All fixed here.

THE COLLAPSE THAT WAS WRONG — installer-migrations. Routing ensureInsideConfig's
containment decision through the realpath-based canonical predicate broke four
tests, and the failure message says it plainly: "migration path escapes
configDir: extensions/gsd.cjs". That module's entire contract is that a
symlinked managed path is snapshotted, restored and backed up AS A LINK and
never dereferenced. The canonical predicate dereferences, then rejects the
result for escaping configDir — so it destroys exactly the thing the module
exists to preserve. Reverted to lexical, with the ruling recorded above the
function so it is not collapsed a third time. normalizeRelPath is the real
pre-gate there; it throws on absolute paths and '..' before this check runs.

That makes THREE deliberately-retained implementations, not two, and they share
one shape worth naming: a realpath-based predicate is the wrong tool wherever a
symlink must be PRESERVED rather than resolved. CONTEXT.md and
docs/explanation/security-model.md are corrected — both previously described
ensureInsideConfig as collapsed.

THE MISSED CONSUMER. tests/security-prompt-injection.security.test.cjs
destructures validatePath from the compiled lib; un-exporting it turned five
tests into TypeError. It appeared in my own earlier search output and I did not
follow it up. Translated under the same rule as the rest: assertions on the
rejection REASON go through assertWithinRoot, boolean-only through
tryWithinRoot.

VALIDATE-ONE-PATH-USE-ANOTHER, FOUND TWICE MORE. This is the fourth and fifth
occurrence in this epic of the exact defect it exists to prevent.
  - scripts/check-glossary-refs.cjs decided containment on `token` and then
    stat'd a separately re-joined path.join(ROOT, token). The ContainedPath is
    now carried through to the probe, so the validated value is the probed one.
  - src/init.cts computed skillPathContained and DISCARDED it, re-joining from
    the raw input for the existsSync and read. The branded type exists to make
    that a type error and here it was inert.

AND THE OVER-CORRECTION OF THAT FIX, caught before it shipped. The first attempt
also substituted the validated value into the EMITTED `ref` for a global skill.
That value is a display token, not a path anything reads through — the only fs
access in that branch runs on the lexical path beforehand — so substituting it
changed emitted output two ways: it is realpath-resolved, so a symlinked global
skills directory would have emitted its resolved target instead of the user's
own path, and it came from path.join, so Windows would have emitted a backslash
where the template has a literal '/'. Restored, with the distinction recorded:
the containment check there is a GATE, not a path producer.

A TEST THAT COULD NOT FAIL. The first symlink regression planted its symlink
from inside a hooked fs.readdirSync and never asserted the planting happened —
if the hook did not fire, the "nothing was written outside" assertion passed
trivially, green against vulnerable code. It now asserts the plant, matching its
sibling. The other two were re-checked: one already asserted its equivalent, the
other plants synchronously and cannot silently no-op.

THE SYMLINK FIX ITSELF, now that the tests are proven red on the matrix.
isPathConfined is lexical by design and structurally cannot see a symlink; three
callers relied on it with no defense of their own. install-engine.cts:1608 and
install-profiles.cts:880 refuse to mkdir/write through a link — mkdirSync with
recursive:true does NOT throw on an existing symlink-to-directory, so a planted
link redirected the SKILL.md write outside the install root.
install-profiles.cts:755 refuses to read through one — statSync FOLLOWS links,
so an outside file's contents were returned and installed as a skill body. Each
mirrors the guard retired-artifact-cleanup.cts:77 already uses.

Severity stated accurately rather than dramatically: only the read at :755 needs
no race. _removeGsdEntries sweeps a pre-planted link at :1608 before the write
loop, and :880's stageDir is a fresh mkdtemp, so both of those require winning a
window. They are fixed as defense-in-depth, not as live exploits.

ALSO: the Changed changeset claimed "every command's observable behavior [is]
unchanged". Three rejection messages are reworded. It now says so, and says that
none of them reveals a host path it previously hid. A stale comment in
verify.cts still named validatePath; an init.cts warning hardcoded "resolves
outside the project directory" for a check that also rejects absolute paths, NUL
bytes and empty strings; and the rationale deleted with check-glossary-refs'
retired helper is restored, noting honestly that a rejected token is now
realpath-resolved before rejection rather than rejected by string comparison.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/init.cts

@@ -4017,9 +4017,9 @@ function buildAgentSkillsBlock(
       }
       const globalSkillMdContained = tryWithinRoot(globalSkillMd, globalSkillsBase, PathAcceptance.AbsoluteInsideRoot);
       if (globalSkillMdContained === null) {
-        const acceptedViaTrustedRoot = trustedGlobalRoots.some((root) => {
-          return tryWithinRoot(globalSkillMd, root, PathAcceptance.AbsoluteInsideRoot) !== null;
-        });
+        const acceptedViaTrustedRoot = trustedGlobalRoots.some(
+          (root) => tryWithinRoot(globalSkillMd, root, PathAcceptance.AbsoluteInsideRoot) !== null,
+        );
         if (!acceptedViaTrustedRoot) {
           warn(
             `[agent-skills] WARNING: Global skill "${skillName}" failed path check (symlink escape?) — skipping\n`,
@@ -4030,6 +4030,13 @@ function buildAgentSkillsBlock(
         // trace, not a skip, so it must not land in the diagnostics warnings[].
         process.stderr.write(`[agent-skills] NOTE: Global skill "${skillName}" accepted via trusted_global_roots (resolves outside the default skills dir)\n`);
       }
+      // `ref` is an emitted display token, not a path anything reads or writes
+      // through — the containment check above is a gate, not a path producer.
+      // Emitting the validated (realpath-resolved, platform-separator) value
+      // instead of this literal broke symlinked skill dirs and Windows output.
+      // The only filesystem read here (existsSync above) already ran on the
+      // lexical path before containment was checked, so ADR-4650's "use the
+      // validated value" rule doesn't apply to this emission.
       validEntries.push({ kind: 'include', ref: `${globalSkillDir}/SKILL.md`, display: displayPath });
       continue;
     }
@@ -4037,12 +4044,13 @@ function buildAgentSkillsBlock(
     const skillPathContained = tryWithinRoot(skillPath, projectRoot);
     if (skillPathContained === null) {
       warn(
-        `[agent-skills] WARNING: Skipping unsafe path "${skillPath}": resolves outside the project directory\n`,
+        `[agent-skills] WARNING: Skipping unsafe path "${skillPath}": not confined to the project directory\n`,
       );
       continue;
     }
 
-    const skillMdPath = path.join(projectRoot, skillPath, 'SKILL.md');
+    // ADR-4650: the validated value is the value used — never re-derive from raw input.
+    const skillMdPath = path.join(skillPathContained, 'SKILL.md');
     if (!fs.existsSync(skillMdPath)) {
       // #2941: if the bare name matches a global skill, hint at the global: prefix.
       // The bare name resolves as project-relative (which doesn't exist), but the