fix: persist mid-loop compaction to stop re-compaction loop and revive episodic

The v3 pipeline compacts session history mid-loop (prune_stage +
final-request guard) but only mutates the run's message buffer, never the
session store. Each turn reloads full history and re-compacts from scratch:
message_tokens climb 129k->156k across turns while every turn compacts back
down to ~60k. The lossy compaction differs per run, degrading the agent.

The same missing persistence stalls episodic memory: the cumulative
compaction count never advances, so the episodic worker's idempotency key
(sessionKey:count) is pinned and every cycle after the first is skipped.
Observed on live traffic: 8 run.completed since deploy, 0 new episodic.

Fixes, all reusing existing machinery (no new store methods, no migrations):

- Bug A: emitSessionCompleted reads cumulative GetCompactionCount (matching
  the legacy v2 path) instead of the per-run counter that resets to 0.
- Bug B/anti-loop: finalize passes state.Prune.MidLoopCompacted into
  maybeSummarize; under pressure it lowers the trigger to a unit-aligned
  threshold (compactionInputCap - overhead, same MaxRequestShare the guard
  uses) so the compaction is PERSISTED via the existing TruncateHistory +
  IncrementCompaction path. Defensive floor prevents over-compaction on
  pathological config; tool-result-only bloat still skips (history-only).
- Bug C: SourceID embeds the count (sessionKey:count) so the eventbus dedup
  key advances per compaction cycle instead of swallowing rapid same-session
  turns within the 5m TTL.

Tests: episodic compaction, maybe_summarize pressure, request budget.
go build (PG + sqliteonly), go vet, go test -race all green.

internal/agent/loop_compact_timeout_test.go

@@ -25,8 +25,9 @@ func TestCompactMessagesInPlace_UsesDefaultTimeout(t *testing.T) {
 		capturingProvider: capturingProvider{response: "Summary of conversation."},
 	}
 	loop := &Loop{
-		provider: provider,
-		model:    "claude-3-5-sonnet",
+		provider:      provider,
+		model:         "claude-3-5-sonnet",
+		contextWindow: 200_000,
 	}
 
 	start := time.Now()
@@ -43,8 +44,9 @@ func TestCompactMessagesInPlace_UsesConfiguredTimeout(t *testing.T) {
 		capturingProvider: capturingProvider{response: "Summary of conversation."},
 	}
 	loop := &Loop{
-		provider: provider,
-		model:    "claude-3-5-sonnet",
+		provider:      provider,
+		model:         "claude-3-5-sonnet",
+		contextWindow: 200_000,
 		compactionCfg: &config.CompactionConfig{
 			TimeoutSeconds: 45,
 		},
@@ -64,8 +66,9 @@ func TestCompactMessagesInPlace_NonPositiveTimeoutFallsBackToDefault(t *testing.
 		capturingProvider: capturingProvider{response: "Summary of conversation."},
 	}
 	loop := &Loop{
-		provider: provider,
-		model:    "claude-3-5-sonnet",
+		provider:      provider,
+		model:         "claude-3-5-sonnet",
+		contextWindow: 200_000,
 		compactionCfg: &config.CompactionConfig{
 			TimeoutSeconds: -1,
 		},