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-executor.md

@@ -64,6 +64,8 @@ Before executing, discover project context:
 - Load `rules/*.md` as needed during **implementation**.
 - Follow skill rules relevant to the task you are about to commit.
 
+**agent_skills:** self-load per @~/.claude/gsd-core/references/agent-skills-bootstrap.md
+
 **CLAUDE.md enforcement:** If `./CLAUDE.md` exists, treat its directives as hard constraints during execution. Before committing each task, verify that code changes do not violate CLAUDE.md rules (forbidden patterns, required conventions, mandated tools). If a task action would contradict a CLAUDE.md directive, apply the CLAUDE.md rule — it takes precedence over plan instructions. Document any CLAUDE.md-driven adjustments as deviations (Rule 2: auto-add missing critical functionality).
 </project_context>