feat: preserve session linkage after git rewrites

[peyton-alt]

Summary

Add CLI-side post-rewrite tracking so Entire preserves local session linkage after Git rewrites instead of relying on fallback relinking later.

This PR:

• adds managed post-rewrite hook support
• adds entire hooks git post-rewrite <rewrite-type> wiring
• remaps local session BaseCommit and AttributionBaseCommit after amend and rebase
• keeps LastCheckpointID unchanged so the checkpoint linkage follows the rewritten commit
• documents the new hook behavior in strategy docs

Scope

Supported rewrite flows in this PR:

• git commit --amend
• git rebase
• interactive rebase variants that route through post-rewrite, including reword, squash, and fixup

Not in scope here:

• git reset
• filter-repo / filter-branch
• remote-only rewrites the CLI does not observe

Why

The goal is to keep Entire tracking native Git commands during an active session. Git already provides deterministic old-SHA -> new-SHA mappings through post-rewrite, so the CLI can preserve linkage locally without depending on UI/Darwin reconciliation or custom non-Git linkage metrics.

Verification

• GOCACHE=/tmp/go-build go test ./cmd/entire/cli/strategy -run 'Test(ParsePostRewritePairs|ShadowStrategy_PostRewrite_)' -count=1 -v
• GOCACHE=/tmp/go-build go test -tags=integration ./cmd/entire/cli/integration_test -run 'TestShadow_(AmendPreservesTrailer|PostRewriteAmendRemapsSessionState|PostRewriteRebaseRemapsSessionState)' -count=1 -v
• Manual claude -p smoke test confirmed a real Entire-tracked commit still gets an Entire-Checkpoint trailer and session state in .git/entire-sessions; the follow-up live amend flow was not used as the primary verifier because Claude hit a write-permission prompt before completing the second turn.

Follow-up

A later PR can add lightweight reset/head-divergence guidance without coupling that to rewrite handling.

---

Note

Medium Risk
Adds a new managed post-rewrite git hook and updates session state/shadow-branch migration logic after amend/rebase, which could affect how checkpoints are linked and recovered across rewritten history.

Overview
Adds first-class support for Git’s post-rewrite hook so Entire can immediately preserve local checkpoint/session linkage after git commit --amend and git rebase.

The CLI now installs and exposes entire hooks git post-rewrite <rewrite-type>, and ManualCommitStrategy.PostRewrite parses old→new SHA mappings from stdin to remap per-worktree session state (BaseCommit and sometimes AttributionBaseCommit) while keeping LastCheckpointID stable.

Shadow-branch migration logic is refactored to a reusable migrateShadowBranchToBaseCommit helper and is reused during rewrites; tests are expanded with new unit + integration coverage for amend/rebase remapping and hook installation/command wiring, and docs are updated to mention post-rewrite behavior.

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[Copilot]

Pull request overview

This PR adds first-class post-rewrite hook handling to keep Entire’s manual-commit session linkage consistent after Git history rewrites (amend/rebase), so sessions continue to track the rewritten commits without relying on later fallback relinking.

Changes:

• Add managed installation support for the post-rewrite Git hook and expose entire hooks git post-rewrite <rewrite-type>.
• Implement ManualCommitStrategy.PostRewrite to remap session BaseCommit / AttributionBaseCommit based on Git-provided old→new SHA pairs.
• Add unit + integration tests covering parsing and session state remapping behavior.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cmd/entire/cli/strategy/manual_commit_hooks.go	Adds PostRewrite handler and post-rewrite input parsing.
cmd/entire/cli/strategy/manual_commit_session.go	Introduces rewrite pair type + helper to remap session commit fields.
cmd/entire/cli/strategy/hooks.go	Adds post-rewrite to managed hook list and installs hook script.
cmd/entire/cli/strategy/hooks_test.go	Verifies managed hook names + hook installation includes post-rewrite.
cmd/entire/cli/hooks_git_cmd.go	Wires post-rewrite subcommand into the hooks command tree.
cmd/entire/cli/hooks_git_cmd_test.go	Ensures the new post-rewrite subcommand is discoverable and has the expected usage.
cmd/entire/cli/root_test.go	Updates hidden-parent command-tree test to use the new hook leaf command.
cmd/entire/cli/strategy/manual_commit_test.go	Adds unit tests for parsing + remapping behavior at the strategy layer.
cmd/entire/cli/integration_test/testenv.go	Adds helper to invoke the post-rewrite hook with explicit mappings.
cmd/entire/cli/integration_test/phase_transitions_test.go	Adds integration tests validating state remap for amend/rebase flows.
CLAUDE.md	Documents the new post-rewrite hook behavior in strategy docs.

[peyton-alt]

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