feat: sqlite session storage (#6594)

This PR:

- Adds retainedTail to compaction entries in the new agent harness so we don't have to walk up the tree for the 2000 tokens before compaction,
- Changes getPathToRoot to getPathToRootOrCompaction to only load until last compaction, as unnecessary to access all nodes where it is called,
- Adds a SQLite storage backend, in a separate packages/session-backend-sqlite, with a migration system and schemas as per on-site discussions: sessions to match session header messages (except for metadata, which I couldn't understand what it's used for or where it gets written, so I omitted it), session_entries for shared entry types as columns plus payload as a json for what remains, session_sequences to represent the append-only, serialized nature of the jsonl files, branch_entries to attribute nodes to branches (relationship one-to-many), and session_materialized with the session info (see /session in TUI) to act as a "cache" or quick-access for costs, message count, token info, labels, session name, and model-thinking-level config (e.g. for fast resume).
- This is compatible with the new agent harness Session abstraction.

packages/coding-agent/docs/session-format.md

@@ -232,10 +232,18 @@ Created when context is compacted. Stores a summary of earlier messages.
 {"type":"compaction","id":"f6g7h8i9","parentId":"e5f6g7h8","timestamp":"2024-12-03T14:10:00.000Z","summary":"User discussed X, Y, Z...","firstKeptEntryId":"c3d4e5f6","tokensBefore":50000}
 ```
 
+Newer harness-generated compactions embed the retained post-compaction context directly on the entry, instead of `firstKeptEntryId`:
+
+```json
+{"type":"compaction","id":"f6g7h8i9","parentId":"e5f6g7h8","timestamp":"2024-12-03T14:10:00.000Z","summary":"User discussed X, Y, Z...","tokensBefore":50000,"retainedTail":[{"role":"user","content":"latest request"},{"role":"assistant","content":[{"type":"text","text":"latest reply"}],"provider":"anthropic","model":"claude-sonnet-4-5","usage":{...},"stopReason":"stop"}]}
+```
+
 Optional fields:
 - `usage`: LLM usage from generating the summary; included in session token and cost totals
+- `retainedTail`: Materialized `AgentMessage[]` kept after compaction. This is optional only for backward compatibility with older sessions. Newer harness-generated compactions include it so we can rebuild context from this checkpoint without walking older entries before the compaction entry.
 - `details`: Implementation-specific data (e.g., `{ readFiles: string[], modifiedFiles: string[] }` for default, or custom data for extensions)
 - `fromHook`: `true` if generated by an extension, `false`/`undefined` if pi-generated (legacy field name)
+- `firstKeptEntryId`: for compatibility with old entry format.
 
 ### BranchSummaryEntry
 
@@ -314,8 +322,9 @@ Entries form a tree:
 1. Collects all entries on the path
 2. If a `CompactionEntry` is on the path:
    - Includes the compaction entry first
-   - Then entries from `firstKeptEntryId` to compaction
-   - Then entries after compaction
+   - If `retainedTail` is present, it acts as a self-contained checkpoint and entries after the compaction are included
+   - Otherwise entries from `firstKeptEntryId` to the compaction are included
+   - Then entries after compaction are included
 3. Preserves non-message entries in the selected range so interactive mode can render them
 
 `buildSessionContext()` builds on that entry list to produce the message list for the LLM:
@@ -323,11 +332,13 @@ Entries form a tree:
 1. Extracts current model and thinking level settings from the full path
 2. Converts selected entries to messages:
    - `message` -> stored `AgentMessage`
-   - `compaction` -> `compactionSummary`
+   - `compaction` -> `compactionSummary` plus `retainedTail` when present
    - `branch_summary` -> `branchSummary`
    - `custom_message` -> `CustomMessage`
    - `custom` -> no context message
 
+This makes newer compactions act like self-contained checkpoints. `retainedTail` is optional only so older sessions that only store `firstKeptEntryId` continue to load correctly.
+
 ## Parsing Example
 
 ```typescript