[codex] Add session list JSON option

[boxp]

What changed

• add --list-sessions to print the current ceeker session list as JSON instead of launching the TUI
• include pane_id in the exported session payload so LLM/tool callers can identify the target tmux pane
• extract shared session-list reading/sorting/refresh logic into ceeker.session-list and reuse it from both CLI and TUI
• add CLI/session-list tests and save the implementation plan under docs/project_docs/llm-session-list-option/plan.md
• document the new JSON mode in both English and Japanese READMEs

Why

ceeker needed a non-TUI access path for LLM usage. Returning the current session list as JSON makes it machine-readable, and exposing pane_id lets downstream callers map a session back to the correct tmux pane.

Impact

• ceeker --list-sessions now exits after printing JSON to stdout
• the command performs one synchronous pane liveness and capture-based refresh before reading sessions
• if tmux refresh fails, ceeker still returns the stored session list
• TUI session ordering now uses the same shared sorting path as the CLI export

Validation

• make ci