feat(#1826): cmdStateRebuild CLI + dry-run + verbose + integration tests + docs (#1830)

Phase 2 of approved feature #1817. Wires the pure `rebuildCore` transition
(Phase 1, #1827) to the `gsd state rebuild` CLI subcommand per ADR-1817
§5 (heavy/manual counterpart to lightweight, auto-triggered `state sync`).

Source changes:
- src/state.cts: implement cmdStateRebuild. Locks via
  readModifyWriteStateMd (real path) or read-only (dry-run). Wires
  phaseInventoryProvider to a real .planning/phases/ disk scan (same
  canonical source buildStateFrontmatter uses). --dry-run emits a
  structured preview without writing. --verbose tees the audit-log
  entries to stderr (treated as data-only per ADR-1577).
- src/state-command-router.cts: register cmdStateRebuild in the
  StateModule interface + add the rebuild handler with --dry-run and
  --verbose flag parsing.
- src/command-aliases.cts: register the state.rebuild canonical +
  'state rebuild' alias + mutation=true (so the manifest covers the
  new subcommand for SDK parity / dispatch hub tests).

Tests (tests/state-rebuild-cli.test.cjs, 5 cases):
- state rebuild with no flags reconciles drifted body + drops orphan
  table rows + appends audit log (end-to-end #1, #2, audit log).
- state rebuild --dry-run computes the diff, writes nothing (criterion #5).
- state rebuild --verbose tees the log; audit-log section still written.
- Running rebuild twice on the just-rebuilt file is byte-identical
  (criterion #6 end-to-end).
- Missing STATE.md produces a clean 'STATE.md not found' message, no
  stack trace (CONTRIBUTING QA matrix).

Verified locally:
- node --test tests/state-rebuild-cli.test.cjs → 5/5 pass
- node --test tests/state-rebuild.test.cjs → 18/18 pass (Phase 1 regression)
- node --test tests/state-transition.test.cjs → 85/85 pass (ADR-1769 regression)

Docs (docs/COMMANDS.md): document `state rebuild [--dry-run] [--verbose]`
with the canonical-command block format used by `state sync` /
`state prune`.

Changeset (.changeset/1817-state-rebuild.md): type=Added, user-facing
description of the new subcommand (closes #1817 epic on merge).

src/state-command-router.cts

@@ -50,6 +50,7 @@ interface StateModule {
   cmdStateValidate(cwd: string, raw: boolean): void;
   cmdStateSync(cwd: string, opts: { verify: string | boolean | null | undefined }, raw: boolean): void;
   cmdStatePrune(cwd: string, opts: { keepRecent: string; dryRun: boolean }, raw: boolean): void;
+  cmdStateRebuild(cwd: string, opts: { dryRun: boolean; verbose: boolean }, raw: boolean): void;
   cmdStateCompletePhase(cwd: string, raw: boolean, phase: string | null | undefined): void;
   cmdStateMilestoneSwitch(cwd: string, milestone: string | null | undefined, name: string | null | undefined, raw: boolean): void;
 }
@@ -190,6 +191,10 @@ function routeStateCommand({ state, args, cwd, raw, error }: RouteStateCommandOp
         const a = parseNamedArgs(args, ['keep-recent'], ['dry-run']);
         state.cmdStatePrune(cwd, { keepRecent: strArg(a, 'keep-recent') || '3', dryRun: a['dry-run'] === true }, raw);
       },
+      rebuild: () => {
+        const a = parseNamedArgs(args, [], ['dry-run', 'verbose']);
+        state.cmdStateRebuild(cwd, { dryRun: a['dry-run'] === true, verbose: a['verbose'] === true }, raw);
+      },
       // complete-phase: CJS-only — no SDK counterpart.
       'complete-phase': () => {
         const a = parseNamedArgs(args, ['phase']);