fix(hooks): skip post-checkout rebuild when PREV_HEAD equals NEW_HEAD (#2421)

[nothariharan]

Fixes #2421

Problem

_CHECKOUT_SCRIPT in graphify/hooks.py assigns PREV_HEAD= and NEW_HEAD= but never reads either variable.

git checkout -b <new> with no start point reports a branch switch (BRANCH_SWITCH=1) while leaving the tree byte-identical — git passes the same SHA as and. That still launched _REBUILD_BODY_CHECKOUT, a full background rebuild (no changed_paths), mutating graphify-out/ seconds later.

The GRAPHIFY_SKIP_HOOK half of #1809 already landed; the same-head short-circuit did not. The unused assignments were the tell.

Fix

After the existing branch-switch guard, short-circuit when the heads match:

# A no-op checkout (e.g. `git checkout -b` with no start point) reports a
# branch switch but leaves the tree unchanged — nothing to rebuild (#2421).
[ "$PREV_HEAD" = "$NEW_HEAD" ] && exit 0

Real branch switches (PREV_HEAD != NEW_HEAD) are unchanged. Rebase/merge/cherry-pick, missing graphify-out/, worktree, and GRAPHIFY_SKIP_HOOK guards are untouched.

Testing

• Added test_checkout_hook_skips_same_head_noop asserting the short-circuit is present in _CHECKOUT_SCRIPT
• uv run pytest tests/test_hooks.py::test_checkout_hook_skips_same_head_noop tests/test_hooks.py::test_hooks_honor_skip_env -q → passed
• python -m tools.skillgen --check → OK (no skill artifacts touched)

Notes

CI runs on Ubuntu. Local Windows full-suite noise (path separators / geteuid / env backends) is unrelated to this one-line hook-template change.

[graphify-labs]

Looks safe to merge — no coupling regressions and no blocking issues, checked against the code graph (not a self-assessment).

Graphify reviewed this change.

Looks safe to merge — no coupling regressions and no blocking issues, checked against the code graph (not a self-assessment).

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Graphify review — findings

This PR modifies the git post-checkout hook script in graphify/hooks.py to add an early exit when PREV_HEAD equals NEW_HEAD, intended to short-circuit the rebuild logic for no-op checkouts (e.g. git checkout -b with no start point). It also adds a new test, test_checkout_hook_skips_same_head_noop, in tests/test_hooks.py that asserts the checkout script contains the head-assignment lines and the new same-head guard string. The surface area is limited to the checkout hook shell template and its corresponding test coverage.

No blocking issues surfaced.

Analysis details — impact, health, verification

Impact & health

Graphify review

Impact — 278 functions depend on the 145 functions this change touches.

Health — grade A; 6 existing hotspot(s) in the area this change touches (pre-existing, not introduced here):

• dispatch_command() — 2 callers, 110 callees (high)
• install() — 25 callers, 6 callees (high)
• dispatch_install_cli() — 2 callers, 31 callees (high)
• uninstall() — 9 callers, 5 callees (high)
• uninstall_all() — 2 callers, 12 callees (high)
• status() — 6 callers, 4 callees (medium)

Verification — 278 functions in the blast radius were not formally verified this run (proofs are advisory here).

Gate & verification

graphify gate

PASS — objectively clean (no health regressions, tests not run — proofs not run this pass (advisory)). Grounded, not self-assessed.

Advisory (not blocking):

• verification_scope: 151 function(s) in the blast radius were not formally verified this run