v0.9.5: session tree (append-only entry journal + `/tree` `/branch` `/fork` `/resume`)

[Hmbown]

Depends on: I3
Folds in: #576 (fork UX).

Goal

Make sessions an append-only entry journal with an in-memory tree projection:
every non-header entry carries id + parentId, the active position is a leafId,
appending creates a child of the leaf, and branching only moves the leaf — it never
rewrites history. Expose /tree, /branch, /fork, and /resume (including
foreign-session import) as user commands, with an in-TUI interactive fork picker.

Why

codewhale already has an append-only log (AppendLog) and prefix stability
(PrefixStabilityManager/frozen_prefix/messages_revision), but branching today
goes through /restore N / revert_turn / a side git-repo snapshot stack that
does rewrite-ish state and is CLI-only. A tree model makes branch/fork/resume
first-class, undoable, and usable from the TUI (#576). It is also the data model
compaction and branch summaries hang off of, so landing the entry shape now (even
with strategies deferred) avoids a second migration.

Current state (codewhale)

• AppendLog, PrefixStabilityManager, frozen_prefix, messages_revision
  (prefix-stable, append-only — the right substrate).
• crates/tui/src/session_manager.rs: JSON sessions in ~/.codewhale/sessions,
  atomic write + fsync + rename, MAX_SESSIONS = 50, crash checkpoints
  (write_session_checkpoint_if_absent), persistence_actor.
• Branching via /restore N / revert_turn / snapshot side-repo.
• Fork is CLI-only today (Feature Request: Improve Fork UX #576).

Target

1. Add id/parentId on every entry and a leafId position; project an in-memory
   tree. Context rebuilds root→leaf.
2. Tree operations as commands: /tree (interactive navigation), /branch,
   /fork (new session from any node, with the in-TUI picker from Feature Request: Improve Fork UX #576), /resume.
3. branch_summary and compaction entries are first-class session entry types
   (data shape lands now; the summarization strategies are deferred).
4. Fork/resume parity: spawn-depth tracking; define a foreign-session import/export
   container so /resume can ingest sessions from other agents.
5. Replace the /restore N / revert_turn / snapshot-side-repo stack with the tree
   model.

Acceptance criteria

• Every session entry has id/parentId; a leafId tracks the active position;
  the in-memory tree projects from the journal.
• /tree, /branch, /fork, /resume work; branching moves the leaf only
  (history is never rewritten).
• /fork has the in-TUI interactive session picker from Feature Request: Improve Fork UX #576.
• Compaction and branch-summary entries fit the entry shape (even if the
  strategies are deferred).
• /restore/revert_turn/snapshot-side-repo stack removed.
• Atomic write, fsync, crash-checkpoint, and MAX_SESSIONS behavior preserved.
• Suite green; just fmt clean.

Non-goals

• Compaction strategies (summarize/streaming/elision/pruning) and image-bitmap
  archival — deferred (see BEYOND doc B1/B2).
• Memory subsystem — deferred (B3).