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.

packages/mosaic/framework/defaults/README.md

@@ -126,7 +126,7 @@ You can still launch runtimes directly (`claude`, `codex`, etc.) — thin runtim
 │   ├── opencode/          ← AGENTS.md, RUNTIME.md
 │   ├── pi/                ← RUNTIME.md, mosaic-extension.ts
 │   └── mcp/               ← MCP server configs
-├── skills/                ← Universal skills (synced from mosaic/agent-skills)
+├── skills/                ← Universal skills (shipped with the framework package)
 ├── skills-local/          ← Local cross-runtime skills
 ├── memory/                ← Persistent agent memory (preserved across upgrades)
 └── templates/             ← SOUL.md template, project templates
@@ -193,11 +193,11 @@ The installer rejects unrecognized flags or positional arguments before making c
 
 ## Universal Skills
 
-The installer syncs skills from `mosaic/agent-skills` into `~/.config/mosaic/skills/`. Install, wizard finalization, and `mosaic update` automatically reconcile every canonical skill into Claude Code's `~/.claude/skills/` directory.
+Canonical skills ship inside the framework package itself; the installer installs them into `~/.config/mosaic/skills/` together with the rest of the framework (there is no separate skills repository). Install, wizard finalization, and `mosaic update` automatically link every canonical skill into Claude Code's `~/.claude/skills/` directory.
 
 ```bash
-mosaic sync                                                   # Full canonical catalog sync
-~/.config/mosaic/tools/_scripts/mosaic-sync-skills --link-only # Re-link only
+mosaic sync                                                   # Relink the full canonical catalog
+~/.config/mosaic/tools/_scripts/mosaic-sync-skills --link-only # Re-link only (same as default)
 mosaic skill list                                              # Show registered, missing, dangling, and foreign entries
 mosaic skill register <name>                                   # Register or repair one canonical Claude link
 mosaic skill unregister <name>                                 # Remove one Mosaic-owned Claude link