feat(coding-agent): Add --session-dir flag for custom session directory

- Add --session-dir CLI flag to specify custom session directory
- SessionManager API: second param of create(), continueRecent(), list(), open()
  changed from agentDir to sessionDir (direct directory, no cwd encoding)
- When omitted, uses default (~/.pi/agent/sessions/<encoded-cwd>/)
- --session now derives sessionDir from file's parent if --session-dir not provided
- list() validates session header before processing files
- Closes #313

Co-authored-by: scutifer <scutifer@users.noreply.github.com>

packages/coding-agent/examples/sdk/11-sessions.ts

@@ -39,8 +39,10 @@ if (sessions.length > 0) {
 	console.log(`\nOpened: ${opened.sessionId}`);
 }
 
-// Custom session directory
+// Custom session directory (no cwd encoding)
+// const customDir = "/path/to/my-sessions";
 // const { session } = await createAgentSession({
-//   agentDir: "/custom/agent",
-//   sessionManager: SessionManager.create(process.cwd(), "/custom/agent"),
+//   sessionManager: SessionManager.create(process.cwd(), customDir),
 // });
+// SessionManager.list(process.cwd(), customDir);
+// SessionManager.continueRecent(process.cwd(), customDir);