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/src/commands/launch.ts

@@ -1450,7 +1450,7 @@ export function registerLaunchCommands(program: Command): void {
   // Direct framework script delegates
   const directCommands: Record<string, { desc: string; script: string }> = {
     init: { desc: 'Generate SOUL.md (agent identity contract)', script: 'mosaic-init' },
-    sync: { desc: 'Sync skills from canonical source', script: 'mosaic-sync-skills' },
+    sync: { desc: 'Link installed skills into runtime homes', script: 'mosaic-sync-skills' },
     bootstrap: {
       desc: 'Bootstrap a repo with Mosaic standards',
       script: 'mosaic-bootstrap-repo',