docs(verify): `same_commit`'s docstring misstates git's abbreviation default

[pbean]

same_commit's docstring justifies its 7-character floor with a claim about git that does not hold:

def same_commit(a: str, b: str) -> bool:
    """Hash equality tolerant of abbreviated forms (>= 7 chars, git's default
    --short length); sessions sometimes report `git rev-parse --short HEAD`."""

There is no fixed "default --short length". With core.abbrev unset git uses auto, which scales the abbreviation with the repository's reachable-object count and clamps upward to 7 only for small repositories. Measured:

• the project test fixture's sandbox repo (~7 reachable objects) → git rev-parse --short HEAD yields 7
• this repository → yields 8 (78c3a748)

So the sentence is accidentally true in the test sandbox and false in the repo the orchestrator actually runs in. Git's documented hard minimum of 4 is a separate mechanism governing explicit core.abbrev / --short=<n> values, and git still lengthens beyond it whenever uniqueness demands.

The 7 is a constant mirroring nothing external — _OBJECT_ID's comment gets this right ("Length floor mirrors same_commit's 7"), so the two sit in the same file disagreeing about where the number comes from.

Impact is documentation-only — no behavior depends on the claim — but it is the kind of premise that gets reused. It was on its way into a test docstring on #647 before being caught, and a related restatement was drafted into a review comment.

Also worth recording while this is open: same_commit currently has zero executable call sites. The only remaining references are prose (verify.py:255, verify.py:346, two test docstrings). Whether it should be retired is a separate question from the wording, and not one this issue proposes to answer.