Compaction secret redaction matches argument names, not values

Deferred from the v0.10.0 release review. Self-disclosed in `.github/meta/harness-review-followups.md`.

Compaction redacts sensitive tool arguments by **argument name** (`isSensitiveArgName`, `packages/opencode/src/session/compaction.ts:62-75`). A secret that appears in a value rather than a recognisably-named argument — inside a URL, or in free text — is not matched.

Because the compaction ledger is replayed into later synthetic prompts, an unmatched secret can be re-sent after the tool output it came from has been pruned.

Name-based matching cannot close this. It needs allowlisted metadata plus value-level secret detection.