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/platform/file-ops.ts

@@ -94,7 +94,8 @@ export function syncDirectory(
       const relPath = relative(relBase, src);
       const dirName = relPath.split('/').pop() ?? '';
 
-      // Skip any .git directory (top-level or nested, e.g. sources/agent-skills/.git)
+      // Skip any .git directory (top-level or nested; defensive — the
+      // framework tree itself is not a git checkout at install time)
       if (options.excludeGit && (dirName === '.git' || relPath.includes('/.git'))) return;
 
       // Skip preserved paths at top level