enhance(#4014): an unreadable directory must not report as an empty one (#4163)

* test(#4014): add failing-first coverage for unreadable-vs-empty directory scope (epic #3473 B4)

* fix(#4014): an unreadable directory must not report as an empty one (epic #3473 B4)

* test(#4014): update hardcoded generateSlugInternal closing-brace line after import shift

src/core-utils.cts's new #4014 import block shifted every subsequent line by
6, moving generateSlugInternal's real closing brace from line 193 to 199.
tests/slug-derivation-drift-guard.test.cjs's MAJOR-1 fixture hardcodes that
line number to plant a synthetic violation immediately after the function's
real body; the guard script itself locates the boundary dynamically via
brace-matching and needed no change.

* docs(#4014): document the unreadable-directory scope signal and add changeset

* docs(#4014): backfill changeset PR number to #4163

* test(#4014): kill pre-existing core-utils.cjs mutation-score gap, unrelated to this issue's diff

---------

Co-authored-by: sim <sim@local>

docs/features/unreadable-directory-scope-signal.md

@@ -0,0 +1,47 @@
+---
+id: 4014
+title: Unreadable-Directory Scope Signal
+group: v1.7.0 Features
+---
+
+**Purpose:** ADR-3473 §8.4 ("failure is a value") applies to filesystem
+listings, not only command argv. `#3885` (B5) gave `roadmap analyze`,
+`gap-checker`, and `init`'s JSON bundles a `context_read_error` /
+`phase_dir_read_error` string naming an unreadable phase directory — but the
+underlying `has_context` / `hasContext` boolean stayed `false` either way,
+so a consumer branching on that boolean alone still cannot tell "genuinely
+no context file" from "could not read the directory at all." This closes
+that gap with a typed signal, reusing ADR-3180's existing frozen `SCOPE`
+enum rather than a new vocabulary.
+
+**`findContextMdIn` (`src/planning-workspace.cts`) now reports its own
+scope.** Called with a directory path, it returns `{ file, files, scope }`
+instead of a bare filename-or-null, and never throws — an unreadable
+directory reports `scope: 'unreadable'` (previously it threw, forcing every
+caller to hand-roll its own `try`/`catch`); a genuinely absent directory
+(`ENOENT`) reports `scope: 'complete'`, the same "real empty" answer as
+today. The array-input call form (an already-read listing) is unchanged.
+
+**Five downstream call sites gain an additive `scope` field, none renamed
+or removed:** `roadmap analyze`'s `AnalyzePhase.context_scope`,
+`gap-checker`'s `phase_dir_scope`, and `init`'s `context_scope` on all three
+JSON bundles (`init plan-phase`, `init phase-op`, `init manager`) —
+including `cmdInitManager`, whose own read failure previously vanished into
+a bare empty `catch {}` with no signal of any kind. `getPhaseFileStats`
+(`src/core-utils.cts`) — the shared listing owner behind `roadmap analyze`
+and `init`'s `has_context` — no longer lets its own failed read get masked
+by an unrelated, already-successful `scanPhasePlans` scope on the same
+phase directory.
+
+**Known limits:**
+- `context_read_error` / `phase_dir_read_error`'s message text is now a
+  fixed "Could not read phase directory `<path>`" rather than embedding the
+  underlying OS errno text — `findContextMdIn`'s directory-string form
+  reports only the `SCOPE` discriminator, not the raw caught error. The
+  field's presence and type are unchanged; only its message detail is
+  coarser than before #4014.
+- `init.cts`'s three call sites call `findContextMdIn` for the scope signal
+  and then still run their own, pre-existing `fs.readdirSync` on the same
+  path for the rest of their output — an intentional, additive-only choice
+  to avoid altering already-complex failure control-flow at those sites,
+  not a performance optimization.