Add committed dashboard-copy parity test (automation vs repoman) #6

Re-filed from rossoctl/agent-skills#47 (originally opened by @rubambiza) after RepoMan was cloned into this repo. Original discussion/history lives there.

Still applies in repoman: only tests/test-vendor-engine.sh exists; no dashboard-copy parity check. Title updated from "automation vs agent-skills" to "automation vs repoman". Closely related to the standalone-skill resync (#5).

The automation-health-dashboard skill vendors a byte-for-byte copy of the
automation repo's dashboard script + lib set. The RepoMan Phase 5 spec
(§Testing, "Dashboard-copy parity") wants a committed parity assertion so
the copy cannot silently drift — a one-time manual check is exactly how it went
stale before (rossoctl/automation#100, surfaced again by clawgenti on PR #46).

Proposed approach: a committed test that fetches automation's copies via
gh api at a pinned ref/SHA (stored as a constant) and diffs them against the
in-repo copies, failing loudly in CI when either side changes without the
other. Preferred over a vendored snapshot, which is just a second manual copy
that can drift the same way. Costs: a CI network dependency (mitigate with a
short cache or a SKIP_PARITY_CHECK offline escape hatch) and a visible
single-line pinned-SHA bump whenever automation's dashboard changes
intentionally.

Deferred out of PR #46 (verbatim-resync scope); fits the RepoMan Phase 6
dashboard-hardening pass.

Assisted-By: Claude Code

Update: the two inherited github-api.sh correctness bugs that were briefly tracked here have been split out to rossoctl/automation#103 and fixed on the Phase 5 automation branch (re-synced here in f039911). This issue now covers only the committed copy-parity guard (Phase 6).