flatten: skills directly at repo root, no skills/ subdir

~/dev/skills/skills/boss/ was visually awkward (skills/skills
read twice) and structurally unnecessary — the skills ARE the
repo's main content, not a sub-collection inside it.

Layout changes:
- boss/SKILL.md moves to the repo root
- skills/README.md (migration notes) moves to docs/migration.md
  where documentation-about-the-system belongs
- install.sh discovers skills via the SKILL.md marker file
  instead of a hardcoded skills/<name>/ path, so future skill
  additions just drop in at the root

Cross-references in boss/SKILL.md (../README.md, ../brainstorm,
etc.) now resolve against the repo root instead of the old
skills/ subdir, which is what they describe: top-level
README is the skill table; sibling skill dirs are the other
migrated skills.

README.md

@@ -70,7 +70,8 @@ plugin.
 
 ## Status
 
-This is the skeleton commit. The skill bodies and agent files
-still need to be migrated from AILang's in-tree
-`~/dev/ailang/skills/` and generalised against this plugin's
-profile schema. That migration is iteration 1.
+Migration from AILang's in-tree `~/dev/ailang/skills/` is in
+progress. The `boss` skill is the landed pilot; the remaining
+seven skills follow. See `docs/migration.md` for the per-skill
+and per-agent migration checklists and the expected final
+layout.