feat(pi): add persistent Mosaic goal controller

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

@@ -51,12 +51,28 @@ Skills are discovered from:
 
 ### Extensions
 
-The Mosaic Pi extension (`~/.config/mosaic/runtime/pi/mosaic-extension.ts`) handles:
+`mosaic pi` loads framework-owned extensions directly from `~/.config/mosaic/runtime/pi/` in this
+order:
 
-- Session start/end lifecycle hooks
-- Active mission detection and context injection
-- Memory routing to `~/.config/mosaic/memory/`
-- MACP queue status reporting
+1. `mosaic-extension.ts` — session lifecycle, mission context, memory routing, lease/mutator gates,
+   and fleet heartbeat reporting.
+2. `goal-extension.ts` — optional persistent `/goal` controller with per-turn and post-compaction
+   checks.
+
+The goal extension is deployed by Mosaic and MUST NOT be copied into `~/.pi/agent/extensions/`.
+Use `/goal set <statement>` (or `/goal <statement>`) to start, then `/goal status`, `/goal pause`,
+`/goal resume`, or `/goal cancel` to control it. An active goal is injected before every model
+request, restored from branch-specific session entries, and considered achieved only after two
+consecutive evidence-bearing reports. Turn and repeated-no-progress limits pause runaway loops.
+Common credential shapes are redacted before controller-owned goal-state entries are persisted or
+displayed; Pi's own model/tool-call history is separate. Goals and reports must contain references
+and pass/fail summaries rather than secrets or raw sensitive output.
+
+Optional bounded launch settings:
+
+- `MOSAIC_GOAL_MAX_TURNS` — autonomous turn limit, default `40`, accepted range `1..500`.
+- `MOSAIC_GOAL_MAX_NO_PROGRESS` — identical no-progress report limit, default `6`, accepted range
+  `1..100`.
 
 ### Sessions