feat(#61): Implement fallback mechanism for auto-compact token limit recovery

- Add FallbackState interface to track message removal attempts
- Implement getLastMessagePair() to identify last user+assistant message pair
- Add executeRevertFallback() to remove message pairs when compaction fails
- Configure max 3 revert attempts with min 2 messages requirement
- Trigger fallback after 5 compaction retries exceed
- Reset retry counter on successful message removal for fresh compaction attempt
- Clean fallback state on session deletion

Resolves: When massive context (context bomb) is loaded, compaction fails and session becomes completely broken. Now falls back to emergency message removal after all retry attempts fail, allowing session recovery.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/hooks/anthropic-auto-compact/index.ts

@@ -8,6 +8,7 @@ function createAutoCompactState(): AutoCompactState {
     pendingCompact: new Set<string>(),
     errorDataBySession: new Map<string, ParsedTokenLimitError>(),
     retryStateBySession: new Map(),
+    fallbackStateBySession: new Map(),
   }
 }
 
@@ -23,6 +24,7 @@ export function createAnthropicAutoCompactHook(ctx: PluginInput) {
         autoCompactState.pendingCompact.delete(sessionInfo.id)
         autoCompactState.errorDataBySession.delete(sessionInfo.id)
         autoCompactState.retryStateBySession.delete(sessionInfo.id)
+        autoCompactState.fallbackStateBySession.delete(sessionInfo.id)
       }
       return
     }
@@ -120,6 +122,6 @@ export function createAnthropicAutoCompactHook(ctx: PluginInput) {
   }
 }
 
-export type { AutoCompactState, ParsedTokenLimitError } from "./types"
+export type { AutoCompactState, FallbackState, ParsedTokenLimitError } from "./types"
 export { parseAnthropicTokenLimitError } from "./parser"
 export { executeCompact, getLastAssistant } from "./executor"