add proactive context-size compaction with a notes-checkpoint turn

CLAUDE.md

@@ -200,6 +200,23 @@ read them à la carte.
 In-flight or recent context that hasn't earned a section yet.
 Prune freely.
 
+- **Just landed:** proactive context-size compaction. `turn::
+  drive_turn` now has two compaction paths. The old *reactive*
+  one (claude prints `Prompt is too long` → `/compact` + retry)
+  is unchanged — by then the session is already past the window
+  and no turn can run on it. The new *proactive* one fires after
+  a clean turn whose last-inference context
+  (`Bus::last_ctx_usage().context_tokens()`) crossed a watermark:
+  `drive_turn` injects one synthetic notes-checkpoint turn
+  (`CHECKPOINT_PROMPT` — "flush durable state into /state now")
+  then runs `/compact`, so the agent can persist in-flight state
+  before the detail collapses into a summary. Watermark is
+  `HIVE_COMPACT_WATERMARK_TOKENS` (default 150k, ~75% of a 200k
+  window; `0` disables). New `maybe_checkpoint_and_compact`
+  helper, all best-effort (a failed checkpoint/compact is a Note,
+  never fails the turn). Both ag3nt + m1nd loops get it for free
+  via the shared `drive_turn`. `docs/turn-loop.md` gained a
+  Compaction section.
 - **Just landed:** dashboard side panel + forge-linked
   config/approvals + per-bucket model stats. (a) Long content
   (file previews, approval diffs, journald logs) opens in a
@@ -608,8 +625,8 @@ Prune freely.
   agent_web_port + collision banner + spawn/rebuild preflight,
   browser notifications, focus-preserving refresh, generalised
   <details data-restore-key> survival, prompt-on-submit pattern.
-- **Open threads:** two-step spawn, notes compaction,
-  unprivileged containers, Bash allow-list, xterm.js. The
+- **Open threads:** two-step spawn, unprivileged
+  containers, Bash allow-list, xterm.js. The
   deployment / gateway / privsep cluster is tracked as
   `area:ops` forge issues. (Landed since this note was first written:
   extra per-agent MCP servers, per-agent send allow-list,