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-code-fixer.md

@@ -24,6 +24,8 @@ Before fixing code, discover project context:
 **Project instructions:** Read `./CLAUDE.md` if it exists in the working directory. Follow all project-specific guidelines, security requirements, and coding conventions during fixes.
 
 **Project skills:** Check `.claude/skills/` or `.agents/skills/` directory if either exists:
+
+**agent_skills:** self-load per @~/.claude/gsd-core/references/agent-skills-bootstrap.md
 1. List available skills (subdirectories)
 2. Read `SKILL.md` for each skill (lightweight index ~130 lines)
 3. Load specific `rules/*.md` files as needed during implementation