[Issue #34] Add conversation lineage foundation and operations

[CC1227871]

Implementation plan for Issue #34

This draft PR is intentionally staged so each layer is reviewable and pushed as a separate commit.

Phase 1 — conversation lineage foundation

• stable branch/message/run/generation identities
• append-only branch projection and reload-safe cursors
• Main branch migration for existing sessions

Phase 2 — conversation operations and IPC

• send, retry failed/cancelled, regenerate, edit, and fork operations
• branch-aware session IPC and renderer client surface
• persisted run manifests and workspace snapshots
• Node smoke coverage for send → regenerate → edit → fork

Phase 3a — native Pi conversation cursors

• typed RPC support for fork, get_fork_messages, and get_entries
• stable runtime entry/leaf identity types for audit and branch isolation
• protocol-level fake-runtime coverage

Phase 3b — native Pi branch isolation

• branch preparation before a child generation
• native Pi fork(entryId) when a stable user entry exists
• fresh branch session file when no native cursor is available
• per-branch runtime state and runtime session cleanup
• runtime user/assistant entry IDs and leaf IDs persisted on Run manifests
• branch-aware RunManager/runtime integration

Phase 4a — branch visibility and switching

• branch metadata cache and reload hydration
• visible current-branch selector in the Copilot panel
• switching reloads only the selected branch projection
• branch switch is disabled during an active generation

Phase 4b — message-level conversation actions

• edit action with inline draft/save/cancel UI
• regenerate, retry, and fork actions on the relevant message/run
• operation results activate and reload the returned branch projection
• failed/cancelled runs are loaded after reload and remain retryable
• live event projection avoids duplicating an already reloaded operation message

Verification so far:

• root TypeScript check passes
• packages/shared and packages/ui TypeScript checks pass
• Node smoke flow passes for send → regenerate → edit → fork
• Node smoke flow passes for native branch preparation and runtime-path isolation
• branch atom test covers switching without transcript mixing
• Bun unit tests are not runnable in this environment because the Bun executable is unavailable

Remaining phases: complete the real Copilot demonstration/evidence flow and run the acceptance checklist end to end.

Related to #34.

[helsome]

Draft 阶段的 foundation 方向是对的，尤其是 append-only lineage、独立 run/generation identity、retry/regenerate/edit/fork 的非破坏语义。当前先保持 Draft，不建议提前合基础层，因为 #34 的关键闭环还没有完成：

• 把 Folio branch projection 真正接到 Pi runtime 的 fork/get_fork_messages/get_entries，确保切 branch 后模型上下文本身隔离，而不只是本地 UI/存储投影隔离；
• branch UI：当前 branch 可见、regenerate alternatives 可切换、failed/cancelled 可正确 retry、历史 branch 可返回；
• reload 后验证 branch/message/run lineage、workspace snapshot、tool/citation/evidence 都恢复且不串 branch；
• 用 Bun 跑 unit/integration + 当前仓库 CI。现在“环境没有 Bun”不能作为 ready-to-merge 的最终验证；
• 按 #34 做真实 Copilot E2E：生成回答 → regenerate → 编辑早期问题 → fork → 两个 branch 分别产生不同 tool/evidence，并证明 reload 后引用和 tool state 仍严格隔离。

这些完成后再 Ready for review。当前不要求推翻已有 3 个 phase，重点是把 runtime/UI/真实 E2E 接成完整闭环。

[helsome]

补充更新一下审核标准：上一轮对 #34 的验收要求偏严。你现在已经补到 Phase 3b / 4a / 4b，runtime branch isolation、branch switching、edit/regenerate/retry/fork 的 UI 与 smoke coverage 都已经形成完整骨架。真实 Copilot 的完整多分支 evidence/citation 演示可以作为后续验证，不再要求一次性做到极致。当前继续保持 Draft 主要是尊重作者状态；等你认为实现 ready、标记 Ready for review 后，只要基础 unit/typecheck/关键 smoke 能通过、没有明显 branch 串线或 destructive overwrite 问题，就可以按可合增量处理。

[helsome]

补充当前贡献验收要求（你这条仍是 Draft，所以先不做强制 Request Changes）：转 Ready for review 前，请把验证材料整理成可复现的测试报告，而不是只写“passes”。至少包含实际命令、Bun/OS、pass/fail 数量、失败基线说明。

另外这条已经修改了 BranchSwitcher、AgentPanel、TurnCard 等可见 UI，Ready 前请贴至少一张实际修改后截图；最好覆盖 branch switching / edit-regenerate-fork 相关界面。已有界面改造优先 Before/After。