fix(herdr): Reject a shared SLICE_DIGEST and tell the caller how to fix an unreadable brief

Summary

Two advisories from #456's review rounds, deferred there (the PR's gates were green and every blocking finding was fixed in-round).

1. compose-briefs.sh accepts a shared SLICE_DIGEST. SLICE_SCOPE and SLICE_PATHS are both rejected when supplied under .shared, since each belongs to one seat. SLICE_DIGEST is the same kind of per-seat derived value, but it is deleted from the merged values before the unused-key check with no .shared guard — so a values file carrying .shared.SLICE_DIGEST is silently accepted and discarded rather than refused as misplaced boundary metadata. Add the guard beside the existing two, with a regression.

2. cli.py unreadable-brief diagnostic names no repair. The handler reports the OSError/UnicodeError and stops; rules/error-handling.md Actionable Messages wants the next step — restore a readable UTF-8 brief or regenerate it with compose-briefs.sh, then re-run apply.

Test plan

• .shared.SLICE_DIGEST is refused with the same shape as .shared.SLICE_PATHS
• The unreadable-brief message names the repair

Start in compose-briefs.sh beside the existing .shared.SLICE_SCOPE and .shared.SLICE_PATHS guards, then inspect cli.py's unreadable-brief handler and rules/error-handling.md. Run the relevant regression tests; done means .shared.SLICE_DIGEST is rejected consistently and the unreadable-brief diagnostic tells callers to restore readable UTF-8 or regenerate with compose-briefs.sh before rerunning apply.