fix(#3135): restore workflows/add-backlog.md — capture --backlog had no workflow to load (#3147)

* fix(#3121): implement commands verb in SDK native registry

- Add commandsList handler — returns sorted JSON array of all registered
  verb strings; satisfies workstream-flag.md + agent tooling discoverability
- Register ['commands', commandsList] in DECISION_ROUTING_STATIC_CATALOG
- Add golden-policy exemption (SDK-only, no CJS mirror needed)
- check.decision-coverage-plan/verify were already registered; commands was the remaining gap

Closes #3121

* fix(#3135): restore workflows/add-backlog.md — capture --backlog had no workflow to load

Root cause: PR #2824 consolidated add-backlog into gsd-capture --backlog and
wired capture.md to delegate to workflows/add-backlog.md via execution_context.
The workflow file was never created (same gap class as reapply-patches.md which
was caught and fixed in the same PR). With no file to load, the agent had no
implementation steps to follow when --backlog was invoked.

Fix:
- Restore get-shit-done/workflows/add-backlog.md with full process from deleted
  commands/gsd/add-backlog.md (phase.next-decimal, ROADMAP write, mkdir, commit)
- Preserve #2280 ordering invariant: ROADMAP entry written before directory
- Fix docs/INVENTORY.md: remove incorrect attribution of --backlog to add-todo.md,
  add add-backlog.md row, bump workflow count 84→85
- Update docs/INVENTORY-MANIFEST.json
- Add regression test: every execution_context @-reference in commands/gsd/*.md
  must resolve to an existing workflow file on disk

Closes #3135

docs/INVENTORY-MANIFEST.json

@@ -104,6 +104,7 @@
       "/gsd-workstreams"
     ],
     "workflows": [
+      "add-backlog.md",
       "add-phase.md",
       "add-tests.md",
       "add-todo.md",

docs/INVENTORY.md

@@ -162,15 +162,16 @@ These six routers are descriptor-only entries that the model picks first; the bo
 
 ---
 
-## Workflows (84 shipped)
+## Workflows (85 shipped)
 
 Full roster at `get-shit-done/workflows/*.md`. Workflows are thin orchestrators that commands reference internally; most are not read directly by end users. Rows below map each workflow file to its role (derived from the `<purpose>` block) and, where applicable, to the command that invokes it.
 
 | Workflow | Role | Invoked by |
 |----------|------|------------|
+| `add-backlog.md` | Add a backlog item to ROADMAP.md using 999.x numbering. | `/gsd-capture --backlog` |
 | `add-phase.md` | Add a new integer phase to the end of the current milestone in the roadmap. | `/gsd-phase` (default) |
 | `add-tests.md` | Generate unit and E2E tests for a completed phase based on its artifacts. | `/gsd-add-tests` |
-| `add-todo.md` | Capture an idea or task that surfaces during a session as a structured todo. | `/gsd-capture` (default), `/gsd-capture --backlog` |
+| `add-todo.md` | Capture an idea or task that surfaces during a session as a structured todo. | `/gsd-capture` (default) |
 | `ai-integration-phase.md` | Orchestrate framework selection → AI research → domain research → eval planning into AI-SPEC.md. | `/gsd-ai-integration-phase` |
 | `analyze-dependencies.md` | Analyze ROADMAP.md phases for file overlap and semantic dependencies; suggest `Depends on` edges. | `/gsd-manager --analyze-deps` |
 | `audit-fix.md` | Autonomous audit-to-fix pipeline — run audit, parse, classify, fix, test, commit. | `/gsd-audit-fix` |