Problem

Claude Code's plugin implements getRestoreCommand() which builds a claude --resume <sessionId> command, allowing sessions to resume with full conversation history. The agent-codex plugin does NOT implement getRestoreCommand(), so session-manager.ts always falls back to a fresh getLaunchCommand() — all conversation context is lost on restore.

Related: #161 explored resume workarounds via context re-injection.

Codex CLI Resume Limitations

Unlike Claude Code (claude --resume <id>), the Codex CLI does not have a --resume or --thread-id flag for CLI mode. True conversation resume requires the app-server's thread/resume JSON-RPC method.

What to implement

Implement getRestoreCommand() as a context re-injection workaround:

1. Reuse findCodexSessionFile() to find the matching JSONL session file for the workspace
2. Extract the thread/session ID from the JSONL filename
3. Extract conversation context from the JSONL (last user prompt or session summary)
4. Build a CLI command with context as the initial prompt: codex [flags] -- "Continue previous session. Context: <summary>"
5. Return null if no session found (falls back to fresh getLaunchCommand())

Reference: packages/plugins/agent-claude-code/src/index.ts lines 732-759 — getRestoreCommand() pattern.

Files to modify

• packages/plugins/agent-codex/src/index.ts — add getRestoreCommand() method
• packages/plugins/agent-codex/src/index.test.ts — add tests

Acceptance criteria

• getRestoreCommand() returns a valid CLI command when a matching session exists
• Command includes the same flags as getLaunchCommand() (permissions, model, reasoning)
• Extracted context is shell-escaped and passed as prompt
• Returns null when no matching session found
• Returns null when workspacePath is missing
• Does not throw on corrupt/missing session files