feat(launch): force-load fleet-critical Pi skills + reconcile skill docs (#555)

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)