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

.changeset/serene-lemurs-march.md

@@ -0,0 +1,5 @@
+---
+type: Changed
+pr: 1868
+---
+**GSD subagents now self-load configured agent_skills regardless of orchestrator bash** — projects that map skills via `.planning/config.json` `agent_skills.<agent-type>` no longer silently lose them on `/gsd-autonomous` or Cursor, where `Skill()`-delegated workflow bash init did not reliably run. Each of the 22 consumer agents queries its own type at init and reads the listed skills, with a dedup guard so runtimes that also inject orchestrator-side (Claude Code) never carry two copies. (#1866)