fix(ci): board-review and changes need full history, not fetch-depth 2

[msgwing]

Problem

PR #436 sat BLOCKED for 10 days (2026-09-10 to 2026-09-20). board-review
was crashing, not failing on content:

subprocess.CalledProcessError: Command ['git', 'diff', '--name-only',
'<base-sha>', '<head-sha>'] returned non-zero exit status 128.

board-review and changes both check out with fetch-depth: 2, on the
assumption (documented in the comment above board-review) that a
pull_request event's github.sha is a merge commit one hop ahead of the
real head, so depth 2 reaches both the head and the base. That holds only
while the branch is at most ~1 commit past its base. PR #436 picked up a
second and third push (a missed regeneration, then a retrigger) after
opening, so its base sha fell outside the shallow history and git diff
failed outright with "bad revision" - not a lint failure, a crash, which
still shows as a red required check and blocks merge exactly as if it were.

changes runs the identical git diff base head, on the identical
assumption, and has not hit this yet only because no branch happened to
reach 3 commits while it mattered.

Fix

fetch-depth: 0 (full history) on both checkouts. Small cost, and the
comment explaining the old depth-2 reasoning is corrected in place rather
than deleted quietly.

Test plan

• python .github/check-workflows.py - clean.
• Reproduced locally against PR docs(alternatives): re-measure the self-hosted proxy shelf, 2026-09-10 #436's actual base/head shas before the fix
  (same exit 128); this is a one-line fetch-depth change with no other
  behavioural difference, so no separate unit test applies.

BOARD: MERGE - the merge path itself was silently blocking a real PR for 10 days; this is a correctness fix to an existing gate, not a new one.
EVIDENCE: python .github/check-workflows.py clean; root cause reproduced against PR #436's job log (git diff --name-only 4242a8d... 13947af9... exit 128, both shas confirmed present on main/the PR branch, so the failure is shallow-history-only, not a real missing commit).