fix(mentions): resolve emphasis/spoiler-wrapped @mentions in CLI (#2526)

[boxed-dev]

Fixes #2526.

Problem

The Rust mention parser (buzz-sdk/src/mentions.rs) only accepted an @ at start-of-string or after ASCII whitespace. Any @mention wrapped in markdown emphasis — **@Name**, *@Name*, _@Name_, (@Name), ||@Name|| — was silently dropped: buzz messages send exited 0 with zero p tags, so nobody was notified and no buzz-acp agent woke.

Agents post exclusively through buzz messages send, so this broke agent handoffs with no signal to anyone — a handoff would stall until a human noticed. Desktop's hasMention.ts fixed the same class of bug months ago (#328); the Rust parser never got parity. This is parser divergence: two implementations of "is this an @mention" giving different answers on the two surfaces that have to agree.

Fix

Bring mentions.rs to behavioural parity with hasMention.ts by widening the two existing predicates against one shared delimiter definition — rather than porting the TS regex, which would create a second hand-maintained list that drifts exactly as the first one did.

• is_mention_lead — accept *, _, (, | as leading delimiters, used at both extract_at_names and extract_at_mentions_with_known. Email locals (user@host) stay excluded.
• is_word_boundary — accept trailing *, _, | so @Will Pfleger** matches the full known name instead of falling through to the single-word tokenizer and yielding will.
• CLI — strip code regions before @name extraction in resolve_content_mentions, mirroring the existing NIP-27 URI path, so an @name inside a code sample no longer fires a false notification.

Tests

The gap that let this ship: of 69 existing mention tests, not one had an emphasis character next to an @. Added:

• buzz-sdk (+7): bold/italic/spoiler/paren leading delimiters, trailing emphasis on multi-word names, and email-still-rejected regression guards.
• buzz-cli (+2): full resolution chain (**@Atlas** / _@Atlas_ / (@Will Pfleger) → resolved pubkeys, proving a p tag is emitted) and the code-region exclusion.

Verification

• cargo test -p buzz-sdk -p buzz-cli — 492 passed
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all --check — clean
• cargo build --release -p buzz-cli — clean
• just test-unit — passed

Scope

Fixes the three parser divergences the issue prescribes. Deliberately out of scope: the optional stderr "zero mentions resolved" warning (better as a focused follow-up — a naive version would spam on every email) and the longer-term unification of the Rust and TS parsers.