fix(projects): deliver issue mentions to managed agents

[ScaleLeanChris]

Fixes #2462.\n\n## Summary\n\n- Publish a Project issue comment as a channel-scoped kind 9 event when it mentions an authorized managed agent.\n- Give buzz-acp the originating issue and repository context.\n- Add buzz issues comment so the agent reply preserves the issue e, repository a, and channel h tags.\n- Preserve pull-request review decisions and monotonic Project comment ordering.\n- Add one core relay-backed regression for the complete delivery contract.\n\nThis supersedes only the #2462 portion of #2498. It preserves that PR's correct kind 9 plus channel h direction, while leaving its unrelated mesh-demo and Builderlab changes separate. Thank you to @Chessing234 for the original investigation and implementation direction.\n\nThe response now gets back to where it once belonged: the originating Project issue.\n\n## Authorization model\n\n- Delivery fails closed unless the repository is bound to a Buzz channel.\n- The relay still authorizes the comment author against that channel.\n- The mentioned agent must be an authorized channel member.\n- buzz-acp retains its channel-scoped subscription.\n- This does not add a global p-tag subscription or turn arbitrary mentions into agent tasks.\n\n## Verification\n\nRebased onto current main at 69107dc. The rebase preserved the newer Projects v3 assignment and discussion behavior.\n\n- cargo check -p buzz-acp -p buzz-cli --tests: passed.\n- cargo test -p buzz-acp authorized_project_issue_mention_round_trips_through_acp_contract -- --ignored --nocapture: 1 passed, 0 failed.\n- just test-unit: all 8 suites passed in 143 seconds.\n- just desktop-test: 4,954 passed, 0 failed.\n- just desktop-build: passed.\n- just desktop-tauri-check: passed.\n- just desktop-tauri-test: main suite 2,440 passed, 0 failed, 15 ignored; all auxiliary suites passed, with one native performance test ignored.\n- just web-build: passed.\n- just mobile-check: formatting unchanged and Flutter analysis found no issues.\n- just mobile-test: 1,417 passed, 0 failed.\n\nThe just ci recipe could not run as one uninterrupted process on this Mac because the Hermit Flutter SDK install and both Rust target trees exceeded the remaining disk space. Every constituent gate from that recipe passed on commit 7d2a045 after generated targets were cleaned between the root and Tauri workspaces.\n\nThe single regression starts an isolated relay, publishes an authorized Project mention, drives it through the production EventQueue, observes exactly one ACP session/prompt with issue and repository context, publishes the response through the real CLI dispatch, and verifies one reply attached with the original h, e, and a tags after a later legacy kind 1 comment.\n\nI also performed a live local round trip with Hermes Agent 0.20.0 on the managed MCP routing branch from #5838: one Project mention produced one Hermes prompt and one attached agent reply on the relay.\n\n## Compatibility and privacy\n\n- Ordinary plaintext Project comments remain compatible.\n- Existing pull-request review decision tags and ordering remain intact.\n- Projects v3 assignment events remain compatible after the current-main rebase.\n- No credentials, local files, or environment values are included in events.\n- No scheduler, file scan, upload path, or new global subscription is introduced.\n