fix(preemptive-compaction): notify user on failure and reduce timeout

Reports from david_66 on Discord: sessions get perceived as 'stuck' when
context usage crosses the 78% threshold. Investigation confirmed two
issues in the preemptive compaction hook:

1. PREEMPTIVE_COMPACTION_TIMEOUT_MS was 120s. While the summarize
   request is in flight, tool.execute.after short-circuits via the
   compactionInProgress guard. A hung summarize blocked the session
   for two full minutes before giving up, which users reasonably
   experience as a hang.

2. On failure (timeout or exception) only a log line was emitted. The
   user had no visibility into why their session was unresponsive or
   why auto-compaction never ran, so a transient upstream error could
   silently leave them well above the threshold with no signal.

Fix:

- Reduce timeout 120s -> 60s. Still gives the upstream a generous
  window, but caps the worst-case perceived hang at one minute.
- Show a warning toast via ctx.client.tui.showToast whenever the
  catch block fires, including the underlying error string so users
  can act (retry, manual /compact, or adjust provider).
- Include providerID/modelID in the Compaction failed log entry so
  wild failures are easier to correlate to a specific target model.

Two existing failure-path assertions were updated to match the new
log shape and a new test covers the toast notification contract.

Discord report: https://discord.com/channels/1452487457085063218/1490536332961906829/1491345441399505037

src/hooks/preemptive-compaction.ts

@@ -8,7 +8,7 @@ import {
 import { resolveCompactionModel } from "./shared/compaction-model-resolver"
 import { createPostCompactionDegradationMonitor } from "./preemptive-compaction-degradation-monitor"
 
-const PREEMPTIVE_COMPACTION_TIMEOUT_MS = 120_000
+const PREEMPTIVE_COMPACTION_TIMEOUT_MS = 60_000
 const PREEMPTIVE_COMPACTION_THRESHOLD = 0.78
 const PREEMPTIVE_COMPACTION_COOLDOWN_MS = 60_000
 
@@ -134,7 +134,25 @@ export function createPreemptiveCompactionHook(
 
       compactedSessions.add(sessionID)
     } catch (error) {
-      log("[preemptive-compaction] Compaction failed", { sessionID, error: String(error) })
+      log("[preemptive-compaction] Compaction failed", {
+        sessionID,
+        providerID: cached.providerID,
+        modelID: cached.modelID,
+        error: String(error),
+      })
+      ctx.client.tui.showToast({
+        body: {
+          title: "Preemptive compaction failed",
+          message: `Context window is above ${Math.round(PREEMPTIVE_COMPACTION_THRESHOLD * 100)}% and auto-compaction could not run. The session may grow large. Error: ${String(error)}`,
+          variant: "warning",
+          duration: 10000,
+        },
+      }).catch((toastError: unknown) => {
+        log("[preemptive-compaction] Failed to show toast", {
+          sessionID,
+          toastError: String(toastError),
+        })
+      })
     } finally {
       compactionInProgress.delete(sessionID)
     }