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/install.sh

@@ -17,7 +17,7 @@ set -Eeuo pipefail
 #   MOSAIC_HOME              — target directory (default: ~/.config/mosaic)
 #   MOSAIC_INSTALL_MODE      — prompt|keep|overwrite (default: prompt)
 #   MOSAIC_ALLOW_MISSING_SEQUENTIAL_THINKING — 1 to bypass MCP check
-#   MOSAIC_SKIP_SKILLS_SYNC  — 1 to skip skill sync
+#   MOSAIC_SKIP_SKILLS_SYNC  — 1 to skip linking skills into runtime homes
 #
 # Flags (CLI args, NOT environment variables — see #869 Point-1 C2):
 #   --allow-inactive-enforcement  Explicit, per-invocation opt-out that lets the
@@ -828,7 +828,7 @@ if [[ -x "$SCRIPTS/mosaic-ensure-excalidraw" ]]; then
 fi
 
 if [[ "${MOSAIC_SKIP_SKILLS_SYNC:-0}" != "1" ]] && [[ -x "$SCRIPTS/mosaic-sync-skills" ]]; then
-  "$SCRIPTS/mosaic-sync-skills" >/dev/null 2>&1 && ok "Skills synced" || warn "Skills sync failed (non-fatal)"
+  "$SCRIPTS/mosaic-sync-skills" >/dev/null 2>&1 && ok "Skills linked into runtime homes" || warn "Skills linking failed (non-fatal)"
 fi
 
 if [[ -x "$SCRIPTS/mosaic-migrate-local-skills" ]]; then