Fix checkpoint data not pushed when agent commits and pushes in same turn

[danielweinmann]

Summary

When an AI agent commits and pushes in the same turn, the session checkpoint data on entire/checkpoints/v1 is never included in that push — it's always one push behind.

Root cause: PostCommit sees the session in ACTIVE state and defers condensation (ACTIVE → ACTIVE_COMMITTED, sets PendingCheckpointID). Condensation only happens later at turn-end (HandleTurnEnd). But by then, PrePush has already pushed the stale entire/checkpoints/v1 branch.

Timeline before this fix:

1. Agent commits → PostCommit: ACTIVE → ACTIVE_COMMITTED (deferred)
2. Agent pushes → PrePush: pushes stale entire/checkpoints/v1
3. Turn ends → HandleTurnEnd: condenses to entire/checkpoints/v1 (too late)

Approach: Instead of condensing mid-session (which would split session data and interfere with ongoing agent work), we record the push remote in session state during PrePush, then push again at turn-end after condensation completes. This preserves the existing condensation timing and works for both scenarios:

• Agent commits + pushes in same turn
• Human pushes in separate tab while agent is active

Changes

• session/state.go: Add PendingPushRemote field to State struct (follows same pattern as PendingCheckpointID)
• strategy/manual_commit_push.go: PrePush calls recordPendingPushRemote() to record the remote on ACTIVE_COMMITTED sessions. flushPendingPush (called via defer in handleTurnEndCondense) pushes the metadata branch and clears the field on all exit paths.
• strategy/manual_commit_hooks.go: Clears PendingPushRemote in InitializeSession on new prompt start (Ctrl-C recovery).
• strategy/manual_commit_push_test.go: 7 tests covering recording, skipping, full flow, cleanup, and the early-return regression.

Ref #275

Test plan

• Enable entire on a test repo with a remote, start an agent session, have the agent commit and push in the same turn — verify entire/checkpoints/v1 is pushed with the condensed data (not stale)
• Verify that a normal human workflow (commit in one terminal, push later) is unaffected — PendingPushRemote stays empty for IDLE sessions
• Simulate Ctrl-C after a push but before turn-end, then start a new prompt — verify PendingPushRemote is cleared and doesn't trigger a stale push
• Verify that disabling push_sessions in settings prevents the turn-end push (the setting is checked inside pushSessionsBranchCommon)