feat(agent): add auto-backgrounding and process management (#17)

* feat(agent): add auto-backgrounding to exec tool

- Add yieldMs parameter to exec tool (default 5s) - commands that don't
  complete within this time automatically run in background
- Create shared process-registry.ts for unified process management
- Refactor process.ts to use shared registry
- Add --debug CLI flag for session message logging
- Signal isolation: backgrounded processes ignore abort signals

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix(session): preserve tool_use/tool_result pairs during compaction

Previously, session compaction simply kept the last N messages, which
could break tool_use/tool_result pairs if the cut point fell between
them. This caused "tool_call_id is not found" errors from the API.

Now compaction finds a safe cut point that starts from either:
- A user message without tool_result
- An assistant message whose tool_use is needed by the next tool_result

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat(session): use Kimi as default model for summary compaction

- Auto-detect MOONSHOT_API_KEY from environment
- Use moonshot-v1-128k (cheaper than k2-thinking)
- Fall back to tokens mode if API key not available

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* docs: add rule to never use git commit --amend

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* docs: clarify git amend rule for immediate fixes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/cli.ts

@@ -9,6 +9,7 @@ type CliOptions = {
   thinking?: string | undefined;
   cwd?: string | undefined;
   session?: string | undefined;
+  debug?: boolean | undefined;
   help?: boolean | undefined;
 };
 
@@ -24,6 +25,7 @@ function printUsage() {
   console.log("  --thinking LEVEL Thinking level");
   console.log("  --cwd DIR        Working directory for commands");
   console.log("  --session ID     Session ID for conversation persistence");
+  console.log("  --debug          Enable debug logging");
   console.log("  --help, -h       Show this help");
 }
 
@@ -67,6 +69,10 @@ function parseArgs(argv: string[]) {
       opts.session = args.shift();
       continue;
     }
+    if (arg === "--debug") {
+      opts.debug = true;
+      continue;
+    }
     if (arg === "--") {
       promptParts.push(...args);
       break;
@@ -110,6 +116,7 @@ async function main() {
     thinkingLevel: opts.thinking as any,
     cwd: opts.cwd,
     sessionId: opts.session,
+    debug: opts.debug,
   });
 
   // If it's a newly created session, notify user of sessionId