fix(#1415): loadConfig provenance + agent-skills diagnostic (Resolution Provenance P2) (#1424)

Implements ADR-1411 P2 / #1415, closing #1366.

Part 1 — config-loader.cts:
- Adds `loadConfigResolved(cwd, options) → ConfigResolution { config, source, degraded }`
  with six tagged branch return paths:
  A1 ws+wsconfig → source:'workstream', degraded:false
  A2 no-ws+config → source:'root', degraded:false
  B  ws requested, wsconfig absent → source:'root', degraded:true (intercepts recursive call)
  C  .planning/ exists, no config → source:'builtin-defaults', degraded:false
  D  no .planning/, global defaults readable → source:'global-defaults', degraded:false
  E  no .planning/, no global → source:'builtin-defaults', degraded:false
- `loadConfigResolved` calls `findProjectRoot` at entry to anchor resolution to the
  nearest .planning/ ancestor (cwd-drift fix, heuristic 4 from P1)
- `loadConfig` becomes a one-line delegation: `return loadConfigResolved(cwd, options).config`
- Exports `loadConfigResolved` in the `export =` block

Part 2 — init.cts cmdAgentSkills:
- Imports `findProjectRoot` from `./project-root.cjs`
- Anchors to project root before loading config (fixes #1366 cwd-drift)
- Uses `loadConfigResolved` for provenance; passes projectRoot to buildAgentSkillsBlock
- Computes `configured` + `reason` (AgentSkillsReason enum): 'resolved' |
  'not_configured' | 'configured_empty' | 'configured_unresolved'
- configured_empty and configured_unresolved emit stderr WARNING; not_configured is silent
- --json IR gains: configured, reason, source, degraded (in addition to existing
  agent_type, block, skills_count, warnings)

Tests (TDD):
- tests/config-loader.test.cjs: 8 new provenance tests (RED before impl, GREEN after)
- tests/agent-skills.test.cjs: 7 new diagnostic tests (RED before impl, GREEN after)
- All 306 tests across 4 suites pass (config-loader:33, agent-skills:72, init:105, workstream:96)

Docs:
- CONTEXT.md: Config Loader Module entry updated with loadConfigResolved interface;
  Resolution Provenance entry notes P2 is now implemented
- docs/CLI-TOOLS.md: --json field reference table added for agent-skills

Closes #1366
Part of #1411

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/fix-1415-loadconfig-provenance.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1424
+---
+**`gsd-tools query agent-skills` no longer silently drops a configured agent's skills under cwd or workstream drift** — `cmdAgentSkills` now anchors to the project root via `findProjectRoot` before loading config, so invoking it from a descendant subdirectory or with a `GSD_WORKSTREAM` that has no scoped config correctly resolves the configured `agent_skills` block. A new `loadConfigResolved(cwd, options) → { config, source, degraded }` function reports provenance alongside the config object: `source` distinguishes `'root' | 'workstream' | 'builtin-defaults' | 'global-defaults'`; `degraded:true` signals a workstream was requested but its config.json was absent. The `--json` IR of `agent-skills` gains four new fields — `configured` (bool), `reason` (`'resolved' | 'not_configured' | 'configured_empty' | 'configured_unresolved'`), `source`, and `degraded` — making silent failures visible and testable. A `configured_empty` or `configured_unresolved` agent emits a `stderr WARNING`; an unconfigured agent stays silent. (Closes #1366. Part of #1411, P2 / #1415.)