test(ledger): a project fold outlives a branch switch and a new commit

[fajarhide]

The host's prompt cache is pinned to the git snapshot it started on, so a checkout or a
new commit means the next session shares no prefix and starts cold, even where the bytes
are identical. The ledger's project scope is keyed on the repository root and never reads
a ref, so it folds across both. That is the one saving in this lane the cache
structurally cannot make, and nothing asserted it: grep -n branch src/ledger/mod.rs
returned a doc comment about something else.

The test primes a project scope from a real repository's project_key, switches branch
and lands a commit under it, then asks a new session id for the same block and requires
the fold.

Real git rather than a hand-built .git directory, because the property is that a real
snapshot change does not move the scope, and a fabricated .git would still pass if
someone made the key shell out to git rev-parse. It runs hermetically:
GIT_CONFIG_GLOBAL and GIT_CONFIG_SYSTEM point at files that do not exist and the
identity is set per process, so the machine's own git config decides nothing.

Verification: make ci green, including smoke_test.sh 70/70. Driven red twice, once
per direction it can be wrong.

Making project_key read .git/HEAD, which is the refactor this exists to catch:

assertion `left == right` failed: the project scope moved with the branch, so every
session after a checkout would open a fresh history
  left: "/var/folders/.../.tmpmnkzui#ref: refs/heads/feature"
 right: "/var/folders/.../.tmpmnkzui#ref: refs/heads/main"

Pointing the second session at another project scope, which proves the fold assertion
carries its own weight:

panicked at src/ledger/mod.rs:1800:14:
the project history has to outlive the snapshot it was recorded on

Closes #739

Greptile Summary

The PR adds a regression test proving that project-scoped ledger folding survives a real Git branch switch and subsequent commit.

• Adds a helper that invokes Git without inherited Git-specific environment configuration.
• Creates a real temporary repository, records project history, changes its snapshot, and verifies a new session still receives the fold.
• Documents the regression fix in the changelog.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/ledger/mod.rs	Adds hermetic Git test infrastructure and branch-switch regression coverage; both previously reported environment-isolation issues are fixed.
changelog.d/739.fixed.md	Accurately documents the newly covered project-ledger behavior.

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