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/skills/catalog.ts

@@ -20,12 +20,6 @@ export function loadSkillsCatalog(mosaicHome: string): SkillEntry[] {
     skills.push(...loadSkillsFromDir(canonicalDir, 'canonical'));
   }
 
-  // Fallback to source repo
-  const sourceDir = join(mosaicHome, 'sources', 'agent-skills', 'skills');
-  if (skills.length === 0 && existsSync(sourceDir)) {
-    skills.push(...loadSkillsFromDir(sourceDir, 'canonical'));
-  }
-
   // Load local skills
   const localDir = join(mosaicHome, 'skills-local');
   if (existsSync(localDir)) {