skeleton: plugin layout + docs, skill/agent migration deferred to iter 1

Establishes the repository structure for the skills plugin:
- README + INSTALL describing the two-layer split (plugin
  mechanics vs per-project profile)
- docs/design, profile-schema, pipeline, agent-template covering
  the universal discipline constants and the profile slot model
- templates/project-profile.yml as a copy-and-fill starting point
- templates/CLAUDE.md.fragment with the baseline orchestrator
  rules a project can import
- install.sh / uninstall.sh wiring skills/ + agents/ into
  ~/.claude/ via idempotent symlinks
- skills/ and agents/ directories empty except for migration
  READMEs; the actual SKILL bodies and agent files migrate
  from ~/dev/ailang/skills/ in the next iteration.

No skill or agent runs yet — this commit only stands up the
structure and documents the substitution model.

skills/README.md

@@ -0,0 +1,45 @@
+# skills/
+
+One subdirectory per skill, each containing a `SKILL.md` plus
+any skill-local resources.
+
+The skeleton commit ships this directory empty. The actual
+skill bodies are migrated from AILang's in-tree
+`~/dev/ailang/skills/` and generalised against the profile
+schema (`../docs/profile-schema.md`) in **iteration 1**.
+
+Expected layout after migration:
+
+```
+skills/
+├── brainstorm/
+│   └── SKILL.md
+├── planner/
+│   └── SKILL.md
+├── implement/
+│   └── SKILL.md
+├── audit/
+│   └── SKILL.md
+├── debug/
+│   └── SKILL.md
+├── fieldtest/
+│   └── SKILL.md
+└── docwriter/
+    └── SKILL.md
+```
+
+Migration checklist per skill:
+
+1. Strip project-specific paths (`docs/specs`, `docs/plans`,
+   `design/INDEX.md`, `crates/`, `bench/`).
+2. Strip project-specific commands (`cargo build`,
+   `bench/check.py`).
+3. Replace literals with profile-slot references in prose.
+4. Strip project vocabulary (`AILang`, `Form A`, `.ail.json`,
+   `Boss`); use the profile's vocabulary slots.
+5. Strip project-specific contracts (honesty-rule,
+   feature-acceptance). These belong in the project's own
+   `CLAUDE.md`, not the plugin.
+6. Verify the body still reads coherently for a generic
+   project — would it make sense in a Python web service?
+   A TypeScript library?