fix(workflow): separate owner attribution from mentions

[skyerus]

Fixes #3638.

What changed

• use the existing relay-trusted actor tag for workflow-owner attribution
• reserve p tags for members explicitly mentioned in message content
• keep explicit mentions of the workflow owner notifying normally (actor + p)
• extract message-tag construction into a pure helper with focused regression coverage

Why

Workflow ownership is provenance, while p tags are notification recipients and agent wake targets. Using the owner as an implicit p recipient caused every workflow-generated card to notify or wake its owner even when the rendered message contained no mention.

Buzz already has the right native primitive: relay-signed delegated events use actor for attribution, and Desktop resolves that actor as the effective author. This change uses that existing path rather than introducing a new tag or workflow option.

Verification

• cargo fmt --all -- --check
• workspace Clippy with all targets/features and warnings denied
• cargo test -p buzz-relay --lib workflow_sink — 19 passed, 1 ignored
• cargo test -p buzz-relay --lib workflow_send_message_separates_actor_from_mentioned_member -- --ignored --nocapture — passed against PostgreSQL
• cargo test -p buzz-workflow — 153 passed, 2 ignored