fix(relay): stop workflow code spans from waking the agents they document

[BradGroux]

A workflow that renders an agent's name inside a code block or inline code span still woke that agent. Workflow text is machine-generated markdown, and agents emit code constantly — a code sample naming a colleague is documentation, not an address.

What happens

resolve_mention_pubkeys in crates/buzz-relay/src/workflow_sink.rs scanned the raw workflow text for @name matches, with no awareness of code regions. A mention inside an inline code span or fenced block therefore produced a real p tag on the emitted kind 9 message, and those p tags gate ACP agent wake (event_mentions_agent), so the agent was pinged for something it was only mentioned in as example code.

The CLI compose path already strips code regions before extracting @name mentions (crates/buzz-cli/src/commands/messages.rs, via buzz_sdk::mentions::strip_code_regions). The relay workflow sink never got the equivalent.

The change

Mask code regions with the existing buzz_sdk::mentions::strip_code_regions before scanning, on both the rendered output and the owner's authored template. The helper is already public in the SDK and buzz-relay already depends on buzz-sdk, so this adds no new dependency and no second masking implementation to keep in step.

One invariant worth calling out: strip_code_regions is not length-preserving, so nothing downstream may map positions from the masked copy back onto the original text. resolve_mention_pubkeys is safe today because only pubkeys (in first-appearance order) escape the function, and the code comment now pins that invariant for future refactors.

Tests

Four new cases in the workflow_sink test module:

• see ping @robby for syntax — inline code span mention wakes no one
• fenced block containing ping @Robby — wakes no one
• `code` then @Robby please — masking must not swallow surrounding prose; the prose mention still wakes
• a mid-line fence (not at a line start) is currently left intact by strip_code_regions; that behavior is pinned so a future SDK change that masks it is a visible decision

Verification

cargo test -p buzz-relay on this branch: all 25 workflow_sink tests pass (21 prior, 4 new). The full package suite runs 1144 tests with 6 pre-existing api::media failures that require a live Postgres pool (Sqlx(PoolTimedOut)); verified identical on pristine main at 213092074 with the fix stashed, so they are unrelated to this change. cargo clippy -p buzz-relay --all-targets and cargo fmt -- --check are clean.

Fixes #7660

[github-actions]

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