docs: retire the stale migration tracker, make layout drift-proof

closes #5

migration.md and the README Status section described an AILang->plugin
migration as "in progress" with "the remaining seven skills" pending —
long since false; the whole roster has landed. The architect flagged
both at the specify cycle close as pre-existing debt.

Rather than re-listing every skill (which is exactly what drifted — the
"Expected layout after migration" ASCII tree had to mirror each new skill
and didn't), the fix removes the enumerating renderings:

- migration.md: reframed as a record of the completed migration. The
  per-skill ASCII layout tree is replaced by the structural rule (every
  top-level SKILL.md dir is a skill; agents live beside it under agents/;
  the repo root is the authoritative roster). The per-skill and per-agent
  authoring checklists are kept verbatim — they retain value for new
  skills. Result: no enumeration, so nothing to drift.
- README Status: "migration in progress / remaining seven follow" ->
  "migration complete"; the doc reference now points at the layout
  convention and authoring checklists, not an "expected final layout".

Decision on the issue's open question (update vs retire): neither pure
form. The tracker's enumerating parts are retired; its still-useful
convention and checklists are preserved drift-proof.

docs/migration.md

@@ -1,7 +1,8 @@
 # Migration
 
-This document tracks the migration from AILang's in-tree
-`~/dev/ailang/skills/` into this plugin.
+This document records the migration of AILang's in-tree
+`~/dev/ailang/skills/` into this plugin — now complete — and the
+layout convention and authoring checklists that came out of it.
 
 Each skill is one top-level directory at the repo root,
 containing a `SKILL.md` plus the agent files it dispatches
@@ -9,60 +10,17 @@ under `agents/`. Agents live with their dispatching skill —
 this is what makes the "no orphan agents" rule structurally
 true rather than only documented.
 
-The `boss` skill is the migration pilot (landed); the remaining
-seven skills follow once the pattern is approved.
+## Repo layout
 
-## Expected layout after migration
+Every top-level directory at the repo root that contains a
+`SKILL.md` is a skill; if it dispatches agents, they live in an
+`agents/` subdirectory beside that `SKILL.md`. The repo root is
+the authoritative list of skills — this document deliberately
+does not enumerate them, so it cannot drift out of sync as
+skills are added or renamed.
 
-```
-~/dev/skills/
-├── README.md
-├── INSTALL.md
-├── install.sh
-├── uninstall.sh
-├── docs/
-├── templates/
-├── boss/                          (pilot: landed)
-│   └── SKILL.md
-├── brainstorm/
-│   └── SKILL.md
-├── specify/
-│   ├── SKILL.md
-│   └── agents/
-│       └── grounding-check.md
-├── planner/
-│   ├── SKILL.md
-│   └── agents/
-│       └── plan-recon.md
-├── implement/
-│   ├── SKILL.md
-│   └── agents/
-│       ├── implement-orchestrator.md
-│       ├── implementer.md
-│       ├── spec-reviewer.md
-│       ├── quality-reviewer.md
-│       └── tester.md
-├── audit/
-│   ├── SKILL.md
-│   └── agents/
-│       ├── architect.md
-│       └── bencher.md
-├── debug/
-│   ├── SKILL.md
-│   └── agents/
-│       └── debugger.md
-├── fieldtest/
-│   ├── SKILL.md
-│   └── agents/
-│       └── fieldtester.md
-└── docwriter/
-    ├── SKILL.md
-    └── agents/
-        └── docwriter.md
-```
-
-`boss` has no agents — it is itself the dispatcher of the
-others, not a dispatcher-of-subagents.
+`boss` is the exception with no `agents/`: it is itself the
+dispatcher of the other skills, not a dispatcher-of-subagents.
 
 `install.sh` walks the repo root, treats every top-level
 directory that contains a `SKILL.md` as a skill, and symlinks