feat(#1866): agent-side self-load of configured agent_skills

Each of the 22 consumer agents now self-loads its configured agent_skills
in its mandatory init step, so .planning/config.json agent_skills.<type>
reaches the agent on every runtime — including Cursor and /gsd-autonomous,
where Skill()-delegated workflow bash init did not reliably execute.

- gsd-core/references/agent-skills-bootstrap.md: shared contract
  (query + Read + dedup guard that skips when <agent_skills> is already
  in the prompt, so Claude's orchestrator-side injection never doubles)
- 22 agents/gsd-*.md: one self-load line naming the agent's own type
- gsd-core/workflows/autonomous.md: note that delegated agents self-load
- tests/agent-skills-bootstrap.test.cjs: regression + parity (CONSUMER_AGENTS
  bijection + fast-check property) — Generative-Fix-Divergence guard
- docs: ADR-1866, CONFIGURATION dual-injection How It Works, INVENTORY
  row, Changed changeset

Closes #1866

docs/INVENTORY.md

@@ -292,6 +292,7 @@ Full roster at `gsd-core/references/*.md`. References are shared knowledge docum
 | `common-bug-patterns.md` | Common bug patterns for code review and verification. |
 | `debugger-philosophy.md` | Evergreen debugging disciplines loaded by `gsd-debugger`. |
 | `mandatory-initial-read.md` | Shared required-reading boilerplate injected into agent prompts. |
+| `agent-skills-bootstrap.md` | Shared agent_skills self-load contract (query + Read + dedup guard) injected into all 22 consumer agents. |
 | `project-skills-discovery.md` | Shared project-skills-discovery boilerplate injected into agent prompts. |
 | `research-documentation-lookup.md` | Shared documentation-lookup protocol (Context7 MCP + guarded CLI fallback) injected into all researcher agents. |
 | `research-philosophy.md` | Shared research philosophy (training-as-hypothesis, honest reporting, investigation-not-confirmation) injected into researcher agents. |