feat(digest): filter routine standards notices and flag stale runs

[twistedmelonman]

The live digest (issue #170, 2026-10-04 run) listed 160 standards rows; 159 were noise. This filters them and makes stale runs visible.

Changes

1. Routine notices are dropped. Exact text "no Markdown files", "no shell files", "no YAML files" or "no workflows", at notice level only. A warning-level row with the same text is kept.
2. A notice with identical text on two or more repos collapses to one line with a repo count. I chose the generic rule over matching the Ubuntu runner notice by name: it is one group_by, and it also covers the next notice GitHub repeats across the fleet. Warnings never collapse.
3. Each repo shows its latest run date. A run older than the commit date of the current standards-check-v1 tag in smartwatermelon/github-workflows is flagged "predates current rules". standards.sh resolves the tag at runtime (ref, then the annotated tag object, then the commit date). If it cannot, the section says "Rules date unknown" with the reason; it does not drop the flag silently.

A repo whose rows were all filtered still appears, with its run date, so a stale run stays visible (this is the nightowlstudiollc/kebab-tax case: its 2026-09-19 run predates the .claude/ rule). Summary counts reflect the rows shown. "Not checked" is unchanged.

Tests

New cases in test-standards.sh, with fixtures for the tag. Known-bad cases: a warning-level "no workflows" row survives; a stale run whose only row was filtered is flagged; an unresolvable tag is reported. run-tests.sh passes (109 assertions) and shellcheck -S info is clean on the three changed shell files.

Does not close any issue.