feat: handle git reset in session hooks

[peyton-alt]

Summary

Adds proactive session linkage reconciliation after git reset and passive divergence warnings when reconciliation isn't possible.

When an active session's BaseCommit no longer matches HEAD and HEAD carries the session's own LastCheckpointID as an Entire-Checkpoint trailer, migrateShadowBranchIfNeeded silently reconciles both BaseCommit and AttributionBaseCommit to HEAD on every hook invocation.

When HEAD moves to a commit without a matching trailer, the system surfaces a divergence warning through entire status and a show-once prepare-commit-msg stderr line.

What reconciliation does

migrateShadowBranchIfNeeded gains a reconcile guard before the existing migrate logic. It fires when:

1. state.BaseCommit != current HEAD
2. state.LastCheckpointID is non-empty
3. HEAD's Entire-Checkpoint trailer matches state.LastCheckpointID

When matched: both BaseCommit and AttributionBaseCommit update to HEAD. The old shadow branch is left untouched (preserves rewind data as a safety net).

When not matched: the existing migrate path runs unchanged (updates BaseCommit only, renames shadow branch, leaves AttributionBaseCommit pinned).

Warning surfaces

When the migrate path fires without reconciliation, AttributionBaseCommit != BaseCommit. Two surfaces notify the user:

• entire status — renders a yellow warning per session card (read-only, no state mutations)
• prepare-commit-msg — prints a single stderr line on next commit, gated by DivergenceNoticeShown flag (cleared when condensation realigns the bases)

Verified scenarios

Scenario A: Reset to known checkpoint (reconcile, no warning)

After condensation:      BaseCommit=96a8d42  AttributionBase=96a8d42  ALIGNED
After migration (pull):  BaseCommit=df45e26  AttributionBase=96a8d42  DIVERGED
After reset + reconcile: BaseCommit=96a8d42  AttributionBase=96a8d42  ALIGNED ✅
Old shadow branch entire/df45e26-e3b0c4 preserved ✅

Scenario B: Reset to unrelated commit (migrate, warning fires)

After condensation:       BaseCommit=fca4f11  AttributionBase=fca4f11  DivNotice=false  ALIGNED
After migration (pull):   BaseCommit=35fe407  AttributionBase=fca4f11  DivNotice=false  DIVERGED
After unrelated reset:    BaseCommit=73a8938  AttributionBase=fca4f11  DivNotice=false  DIVERGED
After prepare-commit-msg: BaseCommit=73a8938  AttributionBase=fca4f11  DivNotice=true   DIVERGED
  → stderr: "entire: session attribution diverged after recent history movement;
             figures may be off until next checkpoint" ✅

Known tradeoffs

Attribution ordering: optimized for common case

There is no single ordering of attribution vs migrate in UserPromptSubmit that is correct for both the normal migrate path (pull/rebase) and the reconcile path (reset to checkpoint):

Ordering	Normal migrate (common)	Reconcile at turn start (rare)
migrate → attribution	❌ Undercounts agent lines (base tree includes pulled content)	✅ Uses reconciled base
attribution → migrate (chosen)	✅ Uses original base tree	❌ Uses stale shadow branch

We chose attribution → migrate → clear because normal migrate is the common case. The reconcile-at-turn-start scenario is rare (requires condensation → HEAD moves → reset → new prompt, all within the LastCheckpointID window). Additionally, reconcile fires more often mid-turn via SaveStep/SaveTaskStep where this ordering does not apply (attribution is only calculated at UserPromptSubmit). The next condensation realigns everything regardless.

Old shadow branch unreachable via GetRewindPoints

After reconcile updates state.BaseCommit to HEAD, GetRewindPoints (keyed by BaseCommit) no longer finds checkpoints on the old shadow branch. This is intentional: the user reset to discard that work, so those checkpoints should not appear in normal rewind. The branch is preserved as a git-level safety net (recoverable via git branch -a | grep entire/), not as active rewind data. A dedicated entire recover command would be the right home for surfacing discarded-segment checkpoints.

Other known limitations (follow-up PRs)

• git commit --amend immediately after reset (before any hook fires): The amend hook checks state.BaseCommit == HEAD which is stale until reconcile runs. Window is narrow — any prompt or tool call triggers reconcile first.
• Stale FilesTouched after reconcile: Includes files from the discarded segment. Low severity — does not cause data corruption, worst case is a minimal/empty checkpoint on next condensation.

Changes

File	What
strategy/manual_commit_migration.go	Reconcile guard in migrateShadowBranchIfNeeded
strategy/manual_commit_hooks.go	Reorder UserPromptSubmit (attribution → migrate → clear) + warnIfAttributionDiverged helper
strategy/manual_commit_condensation.go	Clear DivergenceNoticeShown on condensation
session/state.go	Add DivergenceNoticeShown field
status.go	Passive computeSessionDivergenceWarnings (read-only)
testutil/testutil.go	Add CreateBranch, GitReset helpers

Test plan

• 6 unit tests for reconcile/migrate paths (manual_commit_migration_test.go)
• 3 status tests (status_test.go)
• Manual end-to-end verification of both scenarios with real CLI binary
• mise run fmt && mise run lint — 0 issues
• mise run test:ci — all pass (unit + integration + E2E canary 56/56)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds narrow, status-only handling for git reset-driven session divergence by comparing active session linkage against the current HEAD’s Entire-Checkpoint trailer, auto-reconciling only when the checkpoint match is unambiguous and otherwise emitting a warning.

Changes:

• Resolve HEAD and parse Entire-Checkpoint trailer(s) during entire status rendering.
• Reconcile active session BaseCommit/AttributionBaseCommit only when HEAD checkpoint matches LastCheckpointID; otherwise warn without mutating session state.
• Add unit tests covering safe reset reconciliation and divergence warning behavior.

Reviewed changes

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

File	Description
cmd/entire/cli/status.go	Adds HEAD linkage detection, worktree-path normalization, and session divergence reconciliation + warning rendering in status.
cmd/entire/cli/status_test.go	Adds tests exercising reset reconciliation vs divergence warning behavior.

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[gtrrz-victor]

Tested and is working.