fix: workflow @mentions and Linux notification sounds

[Chessing234]

Summary

• Workflow-emitted **@Name** now gets p tags so ACP can wake the agent (workflow_sink's @-mention parser is a third divergent implementation — workflow-emitted **@Name** emits zero p tags #2686)
• Play notification sounds from blob URLs so WebKitGTK can hear them ([Bug] Linux: notification sounds never play, WebKitGTK can't load media from Tauri's custom URI scheme (MEDIA_ERR_SRC_NOT_SUPPORTED) #2562)
• Keep playing sounds even when the OS toast fails to deliver

Test plan

• cargo test -p buzz-relay --lib markdown_emphasis
• cargo fmt -p buzz-relay -- --check
• cargo clippy -p buzz-relay --lib -- -D warnings
• biome check on touched desktop files
• Mention sound on Linux AppImage / tauri dev

Made with Cursor

[bosoud]

Two notes on the workflow_sink.rs part (I filed #2686):

1. _@Robby_ still wakes no one, and the new test can't see it. The left boundary now accepts _, but extends_name still treats _ as a name-continuation char, so after matching Robby the trailing _ fails the right-boundary check and the mention is rejected. The new test passes anyway because the same string also contains *@Robby*, which matches the same pubkey — and the output is deduplicated, so the assertion can't distinguish "both matched" from "only the * one matched". Standalone repro:

   resolve_mention_pubkeys("_@Robby_ take a look", &[m("Robby", &pk('a'))])  // currently []

   Dropping _ from extends_name fixes it without harming underscore display names — a member literally named will_smith still binds fully because longer names match first (worth pinning with its own test).

2. This adds a fourth hand-maintained copy of the boundary rule. The same delimiter set now lives in hasMention.ts, buzz-sdk/mentions.rs (being consolidated in fix(cli): resolve @mentions wrapped in markdown emphasis and skip code regions #2547 per review there), and twice-diverged here — the drift class that produced [Bug] Bold-wrapped @mentions (**@Name**) send zero p tags from buzz-cli — no notification, exit 0, no warning #2526 in the first place. Once fix(cli): resolve @mentions wrapped in markdown emphasis and skip code regions #2547 lands, buzz_sdk::mentions::is_mention_lead is the shared definition and buzz-relay already depends on buzz-sdk; I have a ready branch converging workflow_sink on it (including the extends_name fix + tests) and am happy to PR it on top of either ordering — whichever lands first.

[Chessing234]

Dropped _ from extends_name so _@Robby_ wakes Robby; added a standalone _@Robby_ case plus a will_smith longest-match pin. Happy to take your buzz-sdk converge PR on top once #2547 lands.