feat(#1596): ship GSD skills via .claude-plugin/plugin.json

Phase B-provide of epic #1258. Adds a build-generated skills/ dir +
a skills manifest field so plugin-installed GSD exposes gsd-core:<skill>
the native Claude Code way. Closes the gap where plugin-only installs
lacked the skill surface because bin/install.js never ran.

- scripts/gen-plugin-skills.cjs: build step converting commands/gsd/*.md
  to skills/gsd-<stem>/SKILL.md via convertClaudeCommandToClaudeSkill
- .claude-plugin/plugin.json: add "skills": "./skills/"
- package.json: add skills to files, gen:plugin-skills to build chain
- tests/issue-766-plugin-manifest.test.cjs: Section H conformance
  (manifest field + dir + frontmatter + count parity) + C2 skills symlink
- docs/adr/766-*.md: dated amendment adding skills surface row
- .changeset/rapid-bears-hum.md: type Added
- skills/: 69 generated gsd-<stem>/SKILL.md files (build-committed)

Closes #1596

skills/gsd-map-codebase/SKILL.md

@@ -0,0 +1,83 @@
+---
+name: gsd-map-codebase
+description: "Analyze codebase with parallel mapper agents to produce .planning/codebase/ documents"
+argument-hint: "[--fast [--focus tech|arch|quality|concerns]] [--query <term>|status|diff|refresh] [area]"
+allowed-tools:
+  - Read
+  - Bash
+  - Glob
+  - Grep
+  - Write
+  - Agent
+---
+
+
+<objective>
+Analyze existing codebase using parallel gsd-codebase-mapper agents to produce structured codebase documents.
+
+Each mapper agent explores a focus area and **writes documents directly** to `.planning/codebase/`. The orchestrator only receives confirmations, keeping context usage minimal.
+
+Output: .planning/codebase/ folder with 7 structured documents about the codebase state.
+</objective>
+
+<execution_context>
+@~/.claude/gsd-core/workflows/map-codebase.md
+</execution_context>
+
+<flags>
+- **--fast**: Lightweight scan mode — spawns one mapper agent instead of four. Accepts an optional `--focus` value: `tech`, `arch`, `quality`, `concerns`, or `tech+arch` (default). Faster and lower-context than the full map.
+- **--query**: Codebase intelligence query mode. Sub-commands: `query <term>`, `status`, `diff`, `refresh`. Requires intel to be enabled in config (`intel.enabled: true`). Runs inline for query/status/diff; spawns an agent for refresh.
+- **(no flag)**: Full parallel map — spawns 4 mapper agents to produce all 7 codebase documents.
+</flags>
+
+<context>
+Arguments: $ARGUMENTS
+
+Parse the first token of $ARGUMENTS:
+- If it is `--fast`: strip the flag, run the scan workflow (passing remaining args including optional --focus).
+- If it is `--query`: strip the flag, run the intel workflow (passing remaining args as the subcommand).
+- Otherwise: pass all of $ARGUMENTS as focus area to the map-codebase workflow.
+
+**Load project state if exists:**
+Check for .planning/STATE.md - loads context if project already initialized
+
+**This command can run:**
+- Before /gsd-new-project (brownfield codebases) - creates codebase map first
+- After /gsd-new-project (greenfield codebases) - updates codebase map as code evolves
+- Anytime to refresh codebase understanding
+</context>
+
+<when_to_use>
+**Use map-codebase for:**
+- Brownfield projects before initialization (understand existing code first)
+- Refreshing codebase map after significant changes
+- Onboarding to an unfamiliar codebase
+- Before major refactoring (understand current state)
+- When STATE.md references outdated codebase info
+
+**Skip map-codebase for:**
+- Greenfield projects with no code yet (nothing to map)
+- Trivial codebases (<5 files)
+</when_to_use>
+
+<process>
+1. Check if .planning/codebase/ already exists (offer to refresh or skip)
+2. Create .planning/codebase/ directory structure
+3. Spawn 4 parallel gsd-codebase-mapper agents:
+   - Agent 1: tech focus → writes STACK.md, INTEGRATIONS.md
+   - Agent 2: arch focus → writes ARCHITECTURE.md, STRUCTURE.md
+   - Agent 3: quality focus → writes CONVENTIONS.md, TESTING.md
+   - Agent 4: concerns focus → writes CONCERNS.md
+4. Wait for agents to complete, collect confirmations (NOT document contents)
+5. Verify all 7 documents exist with line counts
+6. Commit codebase map
+7. Offer next steps (typically: /gsd-new-project or /gsd-plan-phase)
+</process>
+
+<success_criteria>
+- [ ] .planning/codebase/ directory created
+- [ ] All 7 codebase documents written by mapper agents
+- [ ] Documents follow template structure
+- [ ] Parallel agents completed without errors
+- [ ] User knows next steps
+</success_criteria>