feat(launch): force-load fleet-critical Pi skills + reconcile skill docs

Pi workers launched via `mosaic [yolo] pi` never loaded any skill because
buildPiSkillArgs emitted `--no-skills` whenever MOSAIC_PI_SKILL_MODE was
unset (the default everywhere), so maintained `~/.config/mosaic/tools/`
wrappers stayed invisible and workers improvised raw `tmux send-keys` /
`tea` / `gh`. An explicit `--skill` overrides `--no-skills` for that path,
so we now force-load a small fleet-critical set (default: `mosaic-tools`)
on every Pi launch regardless of mode — no full-catalog context bloat.

- launch.ts: add DEFAULT_PI_FORCE_SKILLS + forcedPiSkillArgs(); merge into
  every buildPiSkillArgs() return path (existsSync-guarded → no-op until the
  skill is synced). Override via MOSAIC_PI_FORCE_SKILLS (colon-separated;
  empty string disables).
- launch.spec.ts: deterministic 4th-param injection + force-load coverage.
- runtime/pi/RUNTIME.md: reconcile the "skills load natively" drift with the
  real default-off + force-load + MOSAIC_PI_SKILL_MODE behavior.
- templates/agent/**: fix stale `~/.config/mosaic/rails/` → `tools/` (60
  occurrences across 12 scaffold templates; `rails/` no longer exists).

Companion skill `mosaic-tools` ships in mosaic/agent-skills.
Follow-up (NOT auto-applied): live fleet needs `mosaic-sync-skills` +
launcher upgrade to pick up the new skill on running sessions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QoYiBeKNh3BiYtAJS5Z587

packages/mosaic/framework/runtime/pi/RUNTIME.md

@@ -29,7 +29,21 @@ Pi supports `--models` for Ctrl+P model cycling during a session. Use cheaper mo
 
 ### Skills
 
-Mosaic skills are loaded natively via Pi's `--skill` flag. Skills are discovered from:
+By default the launcher starts Pi with `--no-skills` to keep startup context small, then
+force-loads a small set of fleet-critical skills via explicit `--skill` flags (an explicit
+`--skill` overrides `--no-skills` for that path). The default forced set is `mosaic-tools`
+(the must-use `~/.config/mosaic/tools/` cheatsheet: inter-agent messaging + git wrappers).
+
+Tune skill loading with environment variables:
+
+- `MOSAIC_PI_FORCE_SKILLS` — colon-separated skill dir names to force-load (default: `mosaic-tools`;
+  set to an empty string to disable force-loading). Missing skills are skipped silently.
+- `MOSAIC_PI_SKILL_MODE=all` — link every skill found in `~/.config/mosaic/{skills,skills-local}/`
+  (full catalog; larger context).
+- `MOSAIC_PI_SKILL_MODE=discover` — let Pi discover skills natively (no `--no-skills`), still
+  force-loading the fleet set on top.
+
+Skills are discovered from:
 
 - `~/.config/mosaic/skills/` (Mosaic global skills)
 - `~/.pi/agent/skills/` (Pi global skills)