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/CONFIGURATION.md

@@ -528,7 +528,13 @@ The `Skill` tool is granted to consumer agents deliberately and is instruction-b
 
 ### How It Works
 
-At spawn time, workflows call `gsd-tools query agent-skills <type>` (or legacy `node gsd-tools.cjs agent-skills <type>`) to load configured skills. If skills exist for the agent type, they are injected as an `<agent_skills>` block in the Task() prompt.
+Skills reach a consumer agent through **two cooperating seams** (dual injection — see [ADR 1866](adr/1866-agent-skills-dual-injection-contract.md)):
+
+1. **Orchestrator-side (primary on Claude Code).** At spawn time, workflows call `gsd-tools query agent-skills <type>` (or legacy `node gsd-tools.cjs agent-skills <type>`) in their bash init and interpolate the resulting `<agent_skills>` block into the `Task()`/`Agent()` prompt. This is established across ~25 workflows.
+
+2. **Agent-side self-load (durable fallback).** Each of the 22 consumer agents also self-loads in its own mandatory init step — it runs `gsd_run query agent-skills <its-type>` and `Read`s every listed `SKILL.md` per [`gsd-core/references/agent-skills-bootstrap.md`](../gsd-core/references/agent-skills-bootstrap.md). This is the path that works on every runtime, including **Cursor**, where `Skill()`-delegated workflow bash init does not reliably execute and `/gsd-autonomous` delegates via flat `Skill()` calls.
+
+**Dedup guard.** If an agent's prompt already contains an `<agent_skills>` block (orchestrator already injected one), the agent skips self-load — so on runtimes where both seams run (Claude Code), the prompt never carries two copies. `query agent-skills` is read-only and idempotent: it exits 0 with an empty block when nothing is configured for the type, so self-load is zero-overhead for unconfigured agents.
 
 For project-relative and global personal skills, entries appear as `@`-includes: