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).

docs/COMMANDS.md

@@ -1641,6 +1641,28 @@ node gsd-tools.cjs state sync --verify    # Dry-run: show changes without writin
 
 ---
 
+### `state rebuild [--dry-run] [--verbose]`
+
+Re-derive STATE.md body structure from canonical sources (frontmatter + `.planning/phases/` disk scan). Reconciles `## Current Position` prose with frontmatter, drops orphaned rows from the `**By Phase:**` table, clears template-placeholder field values, and de-duplicates `## Session Continuity Archive` blocks down to the 3 most-recent entries. Every mutation is recorded in a structured `## Rebuild Log` audit section appended to STATE.md (ADR-1817 §3).
+
+Heavier and manual counterpart to the lightweight, auto-triggered `state sync`. The two compose non-overlappingly: `sync` patches three frontmatter fields; `rebuild` reconciles body structure. Per ADR-1817 §4, `rebuild` is idempotent — running it twice on a clean file produces no change.
+
+| Flag | Description |
+|------|-------------|
+| `--dry-run` | Compute the rebuild and emit a structured preview, write nothing |
+| `--verbose` | Tee the audit-log entries to stderr in addition to writing them to STATE.md |
+
+**Prerequisites:** `.planning/STATE.md` exists
+**Produces:** Reconciled `STATE.md` with a `## Rebuild Log` audit entry (only when drift was reconciled)
+
+```bash
+node gsd-tools.cjs state rebuild             # Reconcile body structure
+node gsd-tools.cjs state rebuild --dry-run   # Preview the diff without writing
+node gsd-tools.cjs state rebuild --verbose   # Emit audit-log entries to stderr
+```
+
+---
+
 ### `state planned-phase`
 
 Record state transition after plan-phase completes (Planned/Ready to execute).