Fix resume when local metadata branch is stale + preserve diverged refs

[Soph]

Summary

Two related fixes for entire resume and the underlying ref-advance helper. They compose: the second hardens the helper so the first is fully safe in every scenario.

1. Fix the user-visible bug (resume.go)

promoteRemoteTrackingMetadataBranch returned early whenever the local entire/checkpoints/v1 ref existed — even when origin/entire/checkpoints/v1 was ahead. The committed-checkpoint store used by RestoreLogsOnly and resumeSingleSession only falls back to refs/remotes/origin/... when the local ref is missing, so entire resume printed "session log not available" for checkpoints already in the remote-tracking ref.

Drop the early return so SafelyAdvanceLocalRef runs unconditionally — same pattern FetchMetadataBranch already uses after a real fetch.

2. Protect unpushed local commits in SafelyAdvanceLocalRef (strategy/common.go)

The previous contract overwrote any local ref the target couldn't reach, which silently discarded unpushed local commits whenever a fetch landed sibling commits — e.g. checkpoint metadata produced on another machine sharing the same orphan-style refs (entire/checkpoints/v1, the V2 main ref). The objects survived in the loose-objects pool until git gc, but the branch ref no longer pointed at them.

Tighten the helper so it only sets the ref when the move is non-destructive:

• missing → create
• local at/ahead of target → no-op (existing protection)
• local strictly behind target → fast-forward
• diverged or unrelated history → no-op with a debug log

All three production callers (promoteRemoteTrackingMetadataBranch, FetchMetadataBranch, PromoteTmpRefSafely) sync orphan branches where this is the correct semantic. In the rare divergent case, resume falls through to remote-tracking-tree reads (v1) or the DualCheckpointReader → V1 fallback.

TestFetchV2MainFromURL_UpdatesExistingRef was updated to use the existing advanceV2MainOnTop helper — the prior setup advanced via a second call to createV2MainRef (always an unrelated orphan commit), which was only "working" via the now-removed diverged-overwrite path. The realistic CLI flow produces a descendant commit on top of the previous tip.

Commits

1. Fix resume when local metadata branch is stale — the immediate user-visible fix
2. Simplify resume tests after review — dedupe + use existing agent.AgentTypeClaudeCode constant
3. Preserve diverged local refs in SafelyAdvanceLocalRef — the broader hardening

Test plan

• mise run check passes (fmt, lint, unit, integration, canary)
• Failing-before-fix tests in resume_test.go:
  • TestPromoteRemoteTrackingMetadataBranch_FastForwardsStaleLocal
  • TestResumeFromCurrentBranch_FastForwardsStaleLocalMetadata (reproduces the user-visible "session log not available" message)
• Failing-before-fix tests in strategy/safely_advance_local_ref_test.go:
  • TestSafelyAdvanceLocalRef_Diverged_PreservesLocal
  • TestSafelyAdvanceLocalRef_UnrelatedHistory_PreservesLocal
• Existing TestFetchMetadataBranch_DoesNotRewindLocalAhead still passes
• Existing TestFetchV2MainFromURL_DoesNotRewindLocalAhead still passes
• Manual: in a real repo where local entire/checkpoints/v1 is behind origin, entire resume <branch> restores the session instead of printing "session log not available"

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes entire resume behavior when the local metadata branch (entire/checkpoints/v1) exists but is stale relative to refs/remotes/origin/entire/checkpoints/v1, by ensuring the local ref is safely advanced before reading committed checkpoint metadata.

Changes:

• Removed the early return in promoteRemoteTrackingMetadataBranch so it always attempts to advance the local metadata ref to the remote-tracking hash (while avoiding rewinds when local is ahead).
• Added unit coverage for fast-forwarding a stale local metadata ref.
• Added an end-to-end regression test covering resumeFromCurrentBranch when local metadata is behind remote-tracking metadata.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cmd/entire/cli/resume.go	Always promotes/advances the local metadata ref from origin/... before resume metadata reads to avoid “session log not available” when remote-tracking is ahead.
cmd/entire/cli/resume_test.go	Adds targeted and end-to-end tests reproducing the stale-local-metadata resume failure and validating the fix.