feat: restore checkpoint after any AI response turn

[Basit-Balogun10]

Note: This PR is stacked on #2321. The diff shows +2k lines because GitHub uses main as the base - cross-fork branches can't be set as PR bases (tried using Graphite to track the stack too, same limitation). The actual changes in this PR are a few commits on top of #2321. Please review with that context, or we wait for #2321 to be merged first.

Problem

After an AI turn completes, there's no way to roll back to the git state captured at the end of that turn. If the agent goes down a wrong path you have to manually reset the branch or throw away work — there's no checkpoint-based undo.

Tracked in #724 / #2328.

Changes

Core restore flow

• New checkpoint tRPC router (routers/checkpoint.ts) with a restore procedure that:
  1. Runs RevertCheckpointSaga to reset the worktree to the saved git state
  2. Truncates the session .jsonl to the restore point so the session replays correctly
  3. Collects any orphaned checkpoint refs from the discarded lines and deletes them — no accumulation of stale refs from abandoned future turns
• getSessionInfo(taskRunId) added to AgentService to expose { sessionId, repoPath } without leaking internal types

Per-turn restore button

• buildConversationItems now tracks lastCheckpointId per turn — set when a _posthog/git_checkpoint notification is seen, cleared on each new turn start
• AgentMessage gets a restore button in the top-right corner: active when the turn has a checkpoint, disabled (with tooltip) when it doesn't. Only shown on completed turns.
• SessionUpdateView forwards the new showRestoreButton / onRestoreCheckpoint props down

Checkpoint timeline modal (mod+shift+h)

• CheckpointTimelineModal — command-palette-style dialog listing every checkpoint in the session, newest first, with the first 120 chars of the user message that started that turn and a relative timestamp
• Parses events: AcpMessage[] directly (Option A — no new store or API surface, scoped to the current session by definition)
• Shortcut registered as "checkpoint-timeline" in CONFIGURABLE_SHORTCUT_IDS / DEFAULT_KEYBINDINGS / KEYBOARD_SHORTCUTS so it's user-remappable via the keybindings store — built on top of feat: configurable keyboard shortcuts #2321

Restore confirmation

• RestoreCheckpointDialog — confirmation dialog with an amber warning before the restore runs, so accidental clicks don't immediately drop work
• useRestoreCheckpoint hook wires the full flow: opens the confirmation dialog → calls the tRPC mutation → truncates the in-memory events via sessionStoreSetters.truncateEventsToCheckpoint → shows a success/error toast

How did you test this?

• Ran pnpm --filter code typecheck against tsconfig.node.json and tsconfig.web.json — both exit 0 (pre-existing @posthog/platform / @posthog/agent module errors are unrelated to this change)
• Ran pnpm lint — biome passes clean
• Manually traced the restore flow end-to-end: checkpoint tRPC router logic, JSONL truncation returning orphaned IDs, git ref deletion
• Verified buildConversationItems correctly sets lastCheckpointId for turns that have a _posthog/git_checkpoint notification and null for those that don't
• Checked CheckpointTimelineModal parses a sample events array with multiple turns and renders entries newest-first with correct snippets