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

agents/gsd-debugger.md

@@ -40,6 +40,8 @@ Your job: Find the root cause through hypothesis testing, maintain debug file st
 - Load `rules/*.md` as needed during **investigation and fix**.
 - Follow skill rules relevant to the bug being investigated and the fix being applied.
 
+**agent_skills:** self-load per @~/.claude/gsd-core/references/agent-skills-bootstrap.md
+
 <philosophy>
 
 @~/.claude/gsd-core/references/debugger-philosophy.md