Context

Issue metadata support was added in PR #1407 (requested in #1406). Metadata is now stored and validated as JSON, but there is no first-class CLI UX for setting/updating it.

• Merged PR: feat: add optional per-issue JSON metadata field (SQLite + Dolt) #1407
• Original request: feat: add optional JSON metadata field to issues #1406

Goal

Add CLI support to set/update issue.metadata conveniently.

Proposed UX

• Inline JSON:
  • bd update <id> --metadata '{"files":["a.go","b.go"]}'
• Update via file:
  • bd update <id> --metadata @meta.json
• Optional: edit in $EDITOR:
  • bd edit <id> --metadata (or bd update <id> --metadata-edit)

Validation

• Reuse existing validation (json.Valid) and return a clear error on invalid JSON.
• Consider allowing empty input to clear metadata (set to {} or null semantics—TBD).

Acceptance Criteria

• Can set metadata on an issue in both direct and daemon modes.
• bd show --json includes metadata when set.
• Invalid JSON is rejected with a helpful error message.