Feat/intake conversation resume

[xxy33]

Summary / 概述

The intake REPL (arbor with no subcommand) is a planning chat held before a
run is launched. It lived only in memory: quit before launching and it was gone,
so there was nothing to /resume — only launched runs under .arbor/sessions/
were ever persisted. This PR gives the conversation itself the same durability.

• Auto-saves every turn to <cwd>/.arbor/conversations/<conv_id>/
  (messages.jsonl + meta.json), reusing the checkpoint contract's atomic
  JSONL IO. Best-effort — a failure logs and never breaks the chat.
• arbor --continue / -C: reload the newest unfinished conversation and keep
  chatting (like claude -c). -c is already --config, so the short flag is -C.
• /resume now lists saved conversations (tagged [chat]) alongside launched
  runs; a conversation reloads into the live agent, a run replays the orchestrator.
• On resume (both paths) the prior exchange is replayed to the terminal.
• A conversation is marked launched once it fires LaunchExperiment and is
  then excluded from --continue (the run's own checkpoint takes over).

New module src/cli/intake/conversation_store.py with full unit tests, plus
integration tests driving run_intake with the LLM call and input stubbed.
Docs (outputs-and-resume EN+zh) and both README command tables updated.

Linked issue / 关联 issue

Closes #36

Type of change / 改动类型

• 🐞 Bug fix / 缺陷修复 (fix)
• ✨ Feature / 新功能 (feat)
• 📝 Docs / 文档 (docs)
• ♻️ Refactor / 重构 (refactor)
• ✅ Tests / 测试 (test)
• 🔧 Chore / build / CI (chore)

How was this tested? / 如何验证？

uv run pytest tests/test_intake_conversation_store.py tests/test_intake_repl_autosave.py -q   # 16 passed
uv run pytest          # 430 passed, 6 skipped
uv run ruff check src/cli/intake/ src/cli/commands/run.py tests/test_intake_*.py   # clean

# Manual (editable install):
#   arbor -> chat -> /quit         => saved under .arbor/conversations/
#   arbor -C                       => continues last chat AND replays the prior exchange
#   arbor -> /resume               => lists the conversation tagged [chat] alongside runs

Checklist / 检查清单

• PR title follows Conventional Commits (e.g. fix(config): ...). / PR 标题遵循 Conventional Commits 规范。
• The change is focused and reasonably small. / 改动聚焦且体量合理。
• I ran the relevant tests locally (python tests/test_*.py) and they pass. / 我已在本地运行相关测试并通过。
• Docs / examples updated if behavior or config changed. / 若行为或配置有变更，已同步更新文档/示例（含 README.zh-CN.md）。
• No secrets, API keys, or tokens are included in the diff. / diff 中不包含任何密钥、API key 或 token。