Add a purge / right-to-be-forgotten path across the local ledgers

[JSONbored]

Problem: Found during a third-pass gap audit across Miner Waves 1-4. Every local ledger (claim-ledger, event-ledger, governor-ledger, attempt-log, prediction-ledger) is deliberately insert-only — zero DELETE FROM anywhere in any of them — and none of their CLI wrappers expose a way to purge a specific repo's or miner-login's rows. There's no operator-facing way to fully remove a repo's or a contributor's local history (e.g. after offboarding, a mistaken target, or a privacy request).

Area: AMS / Miner (local stores)

Proposal: Add a gittensory-miner purge --repo <owner/repo> [--miner-login <login>] [--dry-run] [--json] command that deletes every row referencing the given repo (optionally scoped further to a miner login) across claim-ledger, event-ledger, governor-ledger, attempt-log, and prediction-ledger.

Deliverables:

• A purge command deleting matching rows from all five ledgers.
• --dry-run support (matching the existing pattern from Add a --dry-run / simulate mode across the miner CLI #4847) reporting row counts per store before any delete.
• Since this touches claim-ledger/governor-ledger (anti-farming-adjacent surfaces per Epic: Miner Wave 4 — AMS Hardening & Packaging #4825's own scope note), the command must be audit-observable — it logs what was purged (repo, per-store counts, timestamp) to stdout/stderr, so a purge can never be silent.

Acceptance criteria:

• Running the purge command removes every row for the given repo (and login, if specified) from all five ledgers, and reports exactly what was deleted.
• --dry-run makes zero writes and reports what would be deleted.
• A purge is itself observable after the fact (not silent).

[linear-code]

JSO-3839