workflow_sink's @-mention parser is a third divergent implementation — workflow-emitted **@Name** emits zero p tags

Split out of #2526 — noted by @SeanGearin while reviewing #2547.

## Problem

#2526 identified two divergent @-mention parsers (Rust `buzz-sdk/src/mentions.rs` and TS `desktop/src/features/messages/lib/hasMention.ts`). There is a **third**: `resolve_mention_pubkeys` in `crates/buzz-relay/src/workflow_sink.rs`, whose doc comment says it "defines the matching contract" for workflow-emitted kind:9 `p` tags.

Its left-boundary check accepts only start-of-string, whitespace, or `(`:

```rust
let is_left_boundary = |i: usize| i == 0 || chars[i - 1].is_whitespace() || chars[i - 1] == '(';
```

So a workflow-generated `**@Robby**` emits zero `p` tags. Those `p` tags gate ACP agent wake (`event_mentions_agent`, `crates/buzz-acp/src/lib.rs`), which reproduces the exact stalled-handoff failure mode from #2526 on a surface where the text is machine-generated — an agent or template emitting emphasis around a mention is not a user typo, it's routine markdown.

The right side of this parser is already emphasis-tolerant (it rejects only name-continuation chars, `alphanumeric | _`), so only the left boundary needs widening. Note the `_` wrinkle: a trailing `_` after a name is a name-continuation char here, so `_@robby_` has a second, smaller divergence too.

## Suggested direction

After #2547 lands, `buzz-sdk::mentions::is_mention_lead` is the shared definition of "what may precede an `@`" (whitespace, `(`, `*`, `_`, `|`, start). `buzz-relay` already depends on `buzz-sdk`, so `workflow_sink.rs` can call it directly instead of keeping a private boundary rule — that would take the parser count needing manual sync from three to two.

Longer term (already noted in #2526): the Rust and TS implementations should be one contract with shared test vectors, or this class of drift recurs.