feat(workflow): silent agent-invocation action — wake an agent from a scheduled workflow without a visible channel post

## Problem

There is no way for a scheduled workflow to **invoke an agent to do work** — i.e. no first-class "run agent" action, and no *silent* path to wake one. This blocks the natural "wake an agent on a cron to run a check/report" pattern.

Today the workflow action set is `send_message` / `send_dm` / `set_channel_topic` / `add_reaction` / `call_webhook` / `request_approval` / `delay` (`crates/buzz-workflow/src/schema.rs`, `ActionDef`). None of them says "invoke agent X with prompt P." The only way to wake an ACP agent from a scheduled workflow is to `send_message` a plain-text `@mention` — because `p` tags gate ACP wake (`event_mentions_agent`, `crates/buzz-acp/src/lib.rs`). That works, but has two rough edges:

1. **It's not silent.** The wake `@mention` is a visible channel post, so a daily "run this check" produces a visible nudge every day even when the agent then decides there's nothing to report. There's no quiet/DM channel for the wake.
2. **`send_dm` — the one action that could make it quiet — isn't implemented** (proposal #2628, "implement the `send_dm` workflow action (WF-07 TODO)").
3. The mention-wake path is also fragile for machine-generated text: workflow-emitted `**@Name**` (emphasis-wrapped) emits zero `p` tags and wakes nobody (#2686), so authors must hand-avoid markdown around the mention.

## Concrete use case

A user asked an agent to "watch this GitHub issue and tell me daily only if something changed." The durable scheduler exists (workflow `schedule` trigger + cron loop, "fully implemented" per `ARCHITECTURE.md`), but there's no clean way to wire the daily fire to *agent logic* that decides whether to report. The workaround is a scheduled `send_message` that plain-text `@mentions` the agent — functional, but it posts a visible nudge every day regardless of outcome.

## Proposed direction

A workflow action that invokes an agent with a prompt as a first-class step — e.g.

```yaml
- id: run
action: invoke_agent
agent: "@Bumble" # or agent pubkey
prompt: "Run the #2699 watch; report only on change."
```

…delivered on the same channel that gates ACP wake (a `p`-tagged event) but **without** requiring a visible human-readable channel post — the invocation is a wake signal, not chat. If a full new action is too big a step, the minimal unlock is landing `send_dm` (#2628) so the wake can be a quiet DM rather than a channel post.

## Non-goals

- Full agent-orchestration DSL / multi-agent choreography. This is just "fire → wake one agent quietly."

---
Surfaced while building a real daily-watch workflow (agent-side). Related: #2628 (`send_dm` TODO), #2686 (workflow mention parser drops p tags on emphasized names).