feat(3544): Workstream Inventory Builder/Reader split (Phase 3 of #3524)

Phase 3 of the CJS↔SDK hard-seam migration (parent #3524).
Introduces the Builder/Reader pattern for paired Modules with
mixed pure-and-I/O concerns — the template for Phase 4 and
follow-up enhancements that migrate other paired Modules.

Phase 1 and Phase 2 migrated Modules where both sides used
character-equivalent logic. Phase 3 introduces the case where
the pure logic is shareable but the I/O is legitimately per-side.
The Builder/Reader split resolves this:

- The Builder is pure — accepts pre-collected data
  (BuilderInputs struct), returns the typed projection. One
  source of truth; one generator-emitted CJS mirror. Drift
  is structurally impossible.
- The Readers are per-side hand-authored Adapters that do the
  fs reads in their native idiom (currently both sync; either
  side can go async later without touching the Builder), then
  delegate to the Builder.

- sdk/src/workstream-inventory/builder.ts — Builder source.
  170 lines. Pure. Exports buildWorkstreamInventory(inputs),
  isCompletedInventory(status), plus the three typed inventory
  interfaces (WorkstreamPhaseInventory, WorkstreamInventory,
  WorkstreamInventoryList).
- sdk/src/workstream-inventory/builder.test.ts — 18 vitest
  pinning fixtures across all status branches, progress-percent
  clamping, active-marker projection, and isCompletedInventory
  classifier.
- sdk/scripts/gen-workstream-inventory-builder.mjs — generator.
  Captures function bodies via Function.prototype.toString();
  emits with the standard GENERATED FILE banner. Includes a
  small `const relative = path.relative;` preamble in the
  output to handle ESM destructured imports in the compiled
  source.
- sdk/scripts/check-workstream-inventory-builder-fresh.mjs —
  freshness check. Imports the generator function directly
  (rather than duplicating logic) — a cleaner pattern than
  Phase 1/2's approach.
- get-shit-done/bin/lib/workstream-inventory-builder.generated.cjs —
  generator-emitted CJS mirror.
- tests/workstream-inventory-builder-generator.test.cjs — 16
  parity assertions confirming CJS-generated output ==
  SDK source output for every fixture.

- bin/lib/workstream-inventory.cjs: 159 → 132 lines.
  Projection logic gone. `inspectWorkstream` and
  `listWorkstreamInventories` collect BuilderInputs via the
  existing sync fs functions and delegate to the Builder.
  `isCompletedInventory` re-exported from the Builder (its
  signature changed from object→string, but no external
  callers exist so the change is safe).
- sdk/src/query/workstream-inventory.ts: 196 → 143 lines.
  Same shape, sync fs (the SDK was already sync — surprise from
  recon). Types re-exported from the Builder.

- sdk/package.json: gen:workstream-inventory-builder and
  check:workstream-inventory-builder-fresh scripts.
- package.json: proxy for the freshness check.
- .githooks/pre-commit: drift block.
- .github/workflows/test.yml: drift check step.
- CONTEXT.md: amended "Workstream Inventory Module" entry
  to document the Builder/Reader split.
- docs/INVENTORY.md, docs/INVENTORY-MANIFEST.json:
  +1 module count, +1 row for the generated builder.

- Full suite: 9229/9229 pass (baseline 9215 + 14 net new from
  the parity assertions).
- Vitest: 18 Builder fixtures pass.
- Reader shrink: -27 lines on CJS, -53 lines on SDK.
- Net diff (modified files only): +68 / -133 = 65-line
  reduction. New files (Builder, generator, freshness check,
  parity test) add ~600 lines of new structured code.

1. `isCompletedInventory` signature changed from
   isCompletedInventory(inventory: object) to
   isCompletedInventory(status: string). Original CJS exported
   the object form but no external caller passed an object —
   they all passed inventory.status. Verified by grep before
   committing.
2. Generator preamble. The compiled ESM uses
   `import { relative } from 'node:path'`, making `relative`
   a free variable in `buildWorkstreamInventory`. The generator
   emits `const relative = path.relative;` so the captured
   function body works in CJS.
3. Freshness check imports the generator. The freshness check
   imports the generator's buildWorkstreamInventoryBuilderCjs()
   function directly rather than duplicating generation logic.
   Cleaner than Phase 1/2; future generators should follow this.

Shareable via the Builder/Reader pattern in future enhancements:
- frontmatter (pure YAML/markdown parsing)
- plan-scan (pure PLAN.md structure parsing)
- decisions (pure decision-record parsing)
- secrets (regex-based detection in text)
- uat (UAT-criteria parsing)

Structural divergence — different approach needed:
- state — sync vs async file ops; mutation paths differ.
- workstream — lifecycle ops; per-side API surface differs.
- phase, roadmap, init, profile-output, template — large
  surfaces; each its own potential enhancement.

None of these is in scope for Phase 3.

Closes #3544.

docs/INVENTORY-MANIFEST.json

@@ -315,6 +315,7 @@
       "validate-command-router.cjs",
       "verify-command-router.cjs",
       "verify.cjs",
+      "workstream-inventory-builder.generated.cjs",
       "workstream-inventory.cjs",
       "workstream-name-policy.cjs",
       "workstream.cjs",

docs/INVENTORY.md

@@ -423,7 +423,8 @@ Full listing: `get-shit-done/bin/lib/*.cjs`.
 | `validate-command-router.cjs` | Thin CJS subcommand router adapter for `gsd-tools validate` |
 | `verify-command-router.cjs` | Thin CJS subcommand router adapter for `gsd-tools verify` |
 | `verify.cjs` | Plan structure, phase completeness, reference, commit validation |
-| `workstream-inventory.cjs` | Shared workstream inventory projection: state fields, phase/plan/summary counts, roadmap phase count, and active marker |
+| `workstream-inventory.cjs` | Shared workstream inventory projection: state fields, phase/plan/summary counts, roadmap phase count, and active marker — thin orchestrator that delegates pure projection to `workstream-inventory-builder.generated.cjs` |
+| `workstream-inventory-builder.generated.cjs` | GENERATED — pure workstream inventory projection builder; CJS artifact emitted from `sdk/src/workstream-inventory/builder.ts` via `sdk/scripts/gen-workstream-inventory-builder.mjs`; do not edit directly |
 | `workstream-name-policy.cjs` | Canonical workstream name validation (`isValidActiveWorkstreamName`) and slug normalization (`toWorkstreamSlug`); shared by all workstream callers |
 | `workstream.cjs` | Workstream CRUD, migration, session-scoped active pointer |
 | `worktree-safety.cjs` | Worktree-root resolution and non-destructive prune policy decisions; owns W017 health-check logic |