git_commit amends clean trees when the message matches, defeating skip_unchanged_targets on re-runs

Problem

git_commit.rb amends whenever HEAD's message matches the session commit message — even when the working tree is clean:

amend = head_status.success? && head_message.rstrip == commit_message.rstrip
...
args << '--amend' if amend
{ 'changed' => true, 'amended' => amend }

A clean-tree amend produces a new commit SHA with an identical tree, and reports changed: true. Consequences on any fleet re-run:

• every repo comes out ok instead of unchanged, so skip_unchanged_targets never skips the GitHub stages
• git_push_to_remote force-pushes ~70 branches whose trees didn't change
• every open PR's head moves → full CI re-runs fleet-wide (~1,000 jobs) for no-op updates

This is why the 2026-08-12 reference-md re-run rewrote all 71 heads (validated identical trees), and it reproduces trivially: run any session twice back-to-back — the second run amends and force-pushes everything.

Fix

Check git status --porcelain FIRST; when the tree is clean, return { 'changed' => false } regardless of whether HEAD's message matches. The amend path should only apply when there IS something to commit on a branch whose HEAD already carries the session commit.

The existing spec "reports changed: false and leaves HEAD alone when there is nothing to commit" misses this because its no-op scenario uses a message that doesn't match HEAD — add a case where it does.

Start in git_commit.rb and inspect the existing no-op spec, whose current case uses a nonmatching message. Add coverage for a clean tree whose HEAD message matches, then run the relevant spec. Done means it reports changed: false, leaves HEAD unchanged, and does not amend.