Session tree structure with id/parentId linking

- Add TreeNode base type with id, parentId, timestamp
- Add *Content types for clean input/output separation
- Entry types are now TreeNode & *Content intersections
- SessionManager assigns id/parentId on save, tracks leafId
- Add migrateSessionEntries() for v1 to v2 conversion
- Migration runs on load, rewrites file
- buildSessionContext() uses tree traversal from leaf
- Compaction returns CompactionResult (content only)
- Hooks return compaction content, not full entries
- Add firstKeptEntryId to before_compact hook event
- Update mom package for tree fields
- Better error messages for compaction failures

packages/coding-agent/docs/session-tree-plan.md

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+# Session Tree Implementation Plan
+
+Reference: [session-tree.md](./session-tree.md)
+
+## Phase 1: SessionManager Core
+
+- [x] Update entry types with `id`, `parentId` fields (using TreeNode intersection)
+- [x] Add `version` field to `SessionHeader`
+- [x] Change `CompactionEntry.firstKeptEntryIndex` → `firstKeptEntryId`
+- [x] Add `BranchSummaryEntry` type
+- [x] Add `byId: Map<string, ConversationEntry>` index
+- [x] Add `leafId: string` tracking
+- [x] Implement `getPath(fromId?)` tree traversal
+- [x] Implement `getEntry(id)` lookup
+- [x] Implement `getLeafId()` helper
+- [x] Update `_buildIndex()` to populate `byId` map
+- [x] Update `saveMessage()` to include id/parentId (returns id)
+- [x] Update `saveCompaction()` signature and fields (returns id)
+- [x] Update `saveThinkingLevelChange()` to include id/parentId (returns id)
+- [x] Update `saveModelChange()` to include id/parentId (returns id)
+- [x] Update `buildSessionContext()` to use `getPath()` traversal
+
+### Type Hierarchy
+
+```typescript
+// Tree fields (added by SessionManager)
+interface TreeNode { id, parentId, timestamp }
+
+// Content types (for input)
+interface MessageContent { type: "message"; message: AppMessage }
+interface CompactionContent { type: "compaction"; summary; firstKeptEntryId; tokensBefore }
+// etc...
+
+// Full entry types (TreeNode & Content)
+type SessionMessageEntry = TreeNode & MessageContent;
+type CompactionEntry = TreeNode & CompactionContent;
+// etc...
+```
+
+## Phase 2: Migration
+
+- [x] Add `CURRENT_SESSION_VERSION = 2` constant
+- [x] Implement `_migrateToV2()` for v1→v2
+- [x] Update `setSessionFile()` to detect version and migrate
+- [x] Implement `_rewriteFile()` for post-migration persistence
+- [x] Handle `firstKeptEntryIndex` → `firstKeptEntryId` conversion in migration
+
+## Phase 3: Branching
+
+- [x] Implement `branchInPlace(id)` - switch leaf pointer
+- [x] Implement `branchWithSummary(id, summary)` - create summary entry
+- [x] Update `branchToNewFile()` to use IDs (no remapping)
+- [ ] Update `AgentSession.branch()` to use new API
+
+## Phase 4: Compaction Integration
+
+- [x] Update `compaction.ts` to work with IDs
+- [x] Update `prepareCompaction()` to return `firstKeptEntryId`
+- [x] Update `compact()` to return `CompactionResult` with `firstKeptEntryId`
+- [x] Update `AgentSession` compaction methods
+- [x] Add `firstKeptEntryId` to `before_compact` hook event
+
+## Phase 5: Testing
+
+- [ ] Add test fixtures from existing sessions
+- [ ] Test migration of v1 sessions
+- [ ] Test context building with tree structure
+- [ ] Test branching operations
+- [ ] Test compaction with IDs
+- [x] Update existing tests for new types
+
+## Phase 6: UI Integration
+
+- [ ] Update `/branch` command for new API
+- [ ] Add `/branch-here` command for in-place branching
+- [ ] Add `/branches` command to list branches (future)
+- [ ] Update session display to show tree info (future)
+
+## Notes
+
+- All save methods return the new entry's ID
+- Migration rewrites file on first load if version < CURRENT_VERSION
+- Existing sessions become linear chains after migration (parentId = previous entry)
+- Tree features available immediately after migration
+- SessionHeader does NOT have id/parentId (it's metadata, not part of tree)
+- Content types allow clean input/output separation