skills: single install path — canonical skills ship with the framework

Phase D2 of plan 2026-08-19: single package, single install, single command.

The framework installer already treats skills/** as a shipped, manifest-owned
framework subtree, so the folded skills now install into
$MOSAIC_HOME/skills with the rest of the framework — no second repository,
no separate sync step:

- mosaic-sync-skills (bash + powershell): the fetch machinery is gone (clone,
  pull, dirty-state migration, rsync from sources/agent-skills). The script
  now only links installed skills into runtime homes. --link-only is a compat
  no-op; --no-link exits having nothing to do.
- catalog.ts: the sources/agent-skills fallback is dead and removed.
- install.sh, launch.ts, defaults/README.md, README.md, skills/README.md:
  references to the second repo rewritten to describe the shipped path.

Verified: clean install into a fresh MOSAIC_HOME produces 102 skills with no
sources/ directory; the linker then links the selected skills into the four
runtime homes with no git involvement.

README.md

@@ -331,7 +331,7 @@ The framework is the bash-based standards layer installed to every developer mac
 ├── bin/mosaic             ← Unified launcher (claude, codex, opencode, pi, yolo)
 ├── guides/                ← E2E delivery, orchestrator protocol, PRD, etc.
 ├── runtime/               ← Per-runtime configs (claude/, codex/, opencode/, pi/)
-├── skills/                ← Universal skills (synced from agent-skills repo)
+├── skills/                ← Universal skills (shipped with the framework package)
 ├── tools/                 ← Tool suites (orchestrator, git, quality, prdy, etc.)
 └── memory/                ← Persistent agent memory (preserved across upgrades)
 ```