fix(#4652): use the validated path, collapse the duplication, correct two false claims

Seven findings from the two-axis review, all fixed in place.

THE ONE THAT MATTERS: cmdTodoComplete validated sourcePath and targetPath and
then ran every fs call against the RAW strings — existsSync, statSync,
readFileSync, platformWriteSync, unlinkSync, and the dry-run path payload —
never sourceCheck.resolved / targetCheck.resolved. That is the exact
"validate one path, use another" shape ADR-4650 names as the defect this epic
exists to prevent, and it is the same bug this phase had just fixed in
check-command-router. Committed inside the fix for it. All I/O now uses the
resolved paths; user-facing messages still echo the raw filename, never a
resolved absolute path.

A VACUOUS TEST, and the false doc claim it was propping up. The test
"[RED #4327] an absolute path outside the project is rejected" would have
passed with ZERO containment logic: path.join(pendingDir, '/abs/outside/x')
yields <pendingDir>/abs/outside/x — Node does not let a later absolute segment
escape — so the name is FOLDED under the root, passes containment, and simply
404s. The test only ever observed "Todo not found". It now asserts what is
actually true and actually valuable: an absolute name is neutralized, and the
real outside file is not read, not moved, and still present afterward.
docs/CLI-TOOLS.md claimed such a path "is rejected as a usage error", which
was false; it now describes the fold-under-root behavior. Traversal and
embedded separators ARE rejected, and those claims stand.

DUPLICATION THIS EPIC EXISTS TO REMOVE. resolvePath already did
isAbsolute-or-join + validatePath + reject; cmdGapAnalysisPlanPost and
cmdCheckPredicate each re-inlined the identical triplet in the same file. Both
now call resolvePath. Cost, stated rather than hidden: its generic message
replaces the two sites' distinct "phase-dir escapes…" wording. The message
still names the offending input, and one predicate with one message is the
point.

SYMLINK COVERAGE was required by #4652's "Done when" and was missing. Added
for both the todos root and --phase-dir, skipping cleanly on EPERM so the
Windows lanes do not fail where unprivileged symlink creation is disallowed.

Both fast-check properties were UNSEEDED. Seeded now.

The changeset named "check decision-coverage-plan" as a boundary; that is a
caller of the shared resolvePath, which the body never mentioned. Corrected.

DISCLOSED, not hidden: ctx.phaseDir is now always the resolved ABSOLUTE path,
so ${PHASE_DIR} interpolation and the "not found in <targetDir>" message show
an absolute value where a relative --phase-dir previously produced a relative
one. That is an observable output change. A test pins it and
docs/reference/gate-predicates.md states it.

Also regenerated scripts/lib/platform-conformance-tier.generated.cjs and its
macos twin — the new tests changed check-predicate.test.cjs's tier
classification. Caught by npm run lint:ci locally rather than by a bench run.

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

.changeset/sharp-tigers-sing.md

@@ -2,4 +2,4 @@
 type: Security
 pr: 0
 ---
-**Path containment at every boundary that takes a directory or filename from the command line** — `todo complete` followed a traversal name outside the todos root and moved the file it found there, `check predicate --phase-dir` let a blocking gate return a passing verdict on evidence from a directory the caller chose, and `check decision-coverage-plan` / `check gap-analysis.plan-post` both accepted a phase directory outside the project. All four now validate against their managed root and reject with a usage error before touching the filesystem. (#4327, #4354)
+**Path containment at every boundary that takes a directory or filename from the command line** — `todo complete` followed a traversal name outside the todos root and moved the file it found there, `check predicate --phase-dir` let a blocking gate return a passing verdict on evidence from a directory the caller chose, and the shared `resolvePath` helper — used by `check decision-coverage-plan` and `check gap-analysis.plan-post` — accepted a phase directory outside the project. All boundaries now validate against their managed root and reject with a usage error before touching the filesystem. (#4327, #4354)