[shepherding] Pi `/tree` branch isolation via `tdai-memory/branch@1` marker

Is this feature related to a problem? | 该功能需求是否与某个问题相关？请描述

Shepherding issue split out of #1070. Follow-up PR author: @kbob3687-hub. Targets feat/server_team.

Background

Pi is a git-native tool. Developers routinely have multiple worktrees or branches open in parallel — a bugfix on hotfix/x and a feature on feature/y should not contaminate each other's memory. Today the Pi adapter treats one repo as one memory identity, which means switching branches silently mixes context.

#1070 introduced a tdai-memory/branch@1 marker convention to give each Pi branch its own memory identity. We want to reintroduce this as an independent PR because the concept doesn't exist anywhere in the current codebase and is genuinely additive to #1126.

In scope

• Marker convention tdai-memory/branch@1 documented and versioned
• Per-branch memory identity derivation (deterministic, no per-machine drift)
• Migration/fallback behavior when the marker is absent (must not break existing Pi sessions post-#1126)
• Wire-in point: MemoryProxy/src/agent-adapters/pi.ts

Out of scope

• Durable delivery (that's the outbox shepherding issue)
• Cross-branch merge / memory reconciliation UX
• Non-Pi clients (Cursor's session-id problem is tracked separately in #1138)

Acceptance criteria

• Same repo + different branches → different memory identities (integration test)
• Missing marker → falls back to current per-repo behavior with a warning log, not a hard error
• Marker version bump path documented (@1 implies @2 will exist)
• No writes to handler.ts or session/codebuddy/init.ts
• Docs updated: how a Pi user opts into or out of branch isolation

Related

• Closes down the /tree isolation portion of #1070
• Independent of #1126, #1142, and the other three shepherding issues

Suggested branching

Branch name: pi-adapter/tree-branch-isolation against feat/server_team.

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