[Bug] Workflow owner p-tag wakes the owner agent on every send_message

**Describe the bug**

Every workflow `send_message` event includes a `p` tag for the workflow owner,
even when the message contains no `@mention`. Because `buzz-acp` treats any
matching `p` tag as a wake signal, an agent that owns a workflow is activated by
every message the workflow sends.

This conflates non-notifying workflow attribution with recipient notification.
It makes mention-free integration/status cards wake the workflow-owning agent,
causing unnecessary agent turns and making `p` tags unreliable as explicit
recipient evidence.

**Steps to reproduce**

1. Create a workflow owned by an agent identity with a `send_message` step whose
text contains no mention of that agent.
2. Trigger the workflow.
3. Inspect the emitted kind `9` event.
4. Observe a `["p", ""]` tag.
5. Observe that the owner agent is activated despite no explicit mention.

The current source path makes this deterministic:

- `crates/buzz-workflow/src/executor.rs`: `SendMessage` loads
`workflow.owner_pubkey` and passes it to the action sink.
- `crates/buzz-relay/src/workflow_sink.rs`: the sink unconditionally initializes
tags with `p:` for attribution, then separately adds `p` tags
for resolved textual mentions.
- `crates/buzz-acp/src/lib.rs`: `event_mentions_agent` returns true for any
matching `p` tag.

This was reproduced on hosted Buzz with two webhook workflows: all eight
observed post-cutover workflow cards carried the owner-agent `p` tag, including
ordinary non-alert events with no textual mention.

**Expected behavior**

Workflow ownership/attribution should not itself notify or activate the owner.
The owner should remain attributable through a dedicated non-notifying tag (or
equivalent metadata), while `p` tags should represent explicit recipients such
as resolved `@mentions`.

An alternative compatible design would be an explicit `send_message` option
that suppresses owner notification while retaining separate attribution.

**Version and platform**

- Buzz version: hosted deployment; source path also present at
`dd222a509b156ba52ed3219e895d7bf1cf322c92`
- OS: server-side workflow/relay/ACP interaction; client OS independent

**Logs / additional context**

No exact duplicate was found after searching open and closed issues for
`workflow owner`, `p-tag`, `workflow attribution`, `workflow notification`,
`workflow mention`, and `agent wake`.

Related but not duplicate:

- #2686 covers the opposite direction: workflow textual mentions failing to
emit required `p` tags in one formatting case.
- #2702 proposes a first-class silent agent-invocation action.

Suggested regression coverage:

1. A mention-free workflow message has owner attribution but no owner `p` tag.
2. A workflow message with one explicit `@Agent` mention has exactly that
recipient `p` tag and wakes only that agent.
3. A workflow owned by the explicitly mentioned agent does not duplicate tags.