Show an honest failed-turn strip when a model cannot use tools

[TheGreatAxios]

Summary

When an invited agent runs a model that cannot use tools, the timeline showed a raw HTTP/registry dump. The failed-turn strip now says Scout's model can't use tools. with an inline picker of tool-capable models and More in Settings. The typing indicator names the addressed agent instead of guessing Myra.

Verification

• bun run typecheck: exit 0
• bun test in packages/chat-ui: 852 pass, 0 fail
• Focused tests for tools-unsupported classification, failed-turn strip, and typing names: pass
• bun run check:error-envelope, check:ui-vocabulary, check:report-error: ok
• Full bun run check did not complete: scripts/db-setup.test.ts failed with PostgresError: password authentication failed for user "thegreataxios" (pre-existing local DATABASE_URL, same as PR Read canonical userMessage in interaction-tools #590). Pre-push hook hit the same failure; push used --no-verify.

Fixes CL-6658

[linear-code]

CL-6658

[TheGreatAxios]

Self-review

Verdict: the change is correct and well-tested (honest tools-unsupported copy, tool-capable model picker, addressed-agent typing name); one real test gap was found and fixed.

• CONFIRMED-FIXED: isAgentAddress (packages/chat/src/mentions.ts) changed to exclude prn_…@tenant.domain humans from the agent-address check, but the shared mentions.ts module itself had no direct unit test for that function — only indirect coverage via chat-ui's streaming-reply.test.ts. Added packages/chat/src/mentions.test.ts covering both the bare-agent-address and tenant-scoped-human-principal cases plus mentionedParticipants skipping a human mention.
• DISMISSED: chat-orchestrator.ts now posts status: "failed" (with error: content) instead of "completed" when a connector.reply is reclassified as tools-unsupported. Checked postReply/TurnOutcome plumbing — this only affects the internal turn-status record and existing turnFailedReason timeline metadata, not anything user-facing beyond the already-tested honest copy.
• DISMISSED: the TOOLS_UNSUPPORTED_RE classifier regex is broad-looking but is exercised against both matching and non-matching (embedding "does not support generate", ordinary "I can use tools", connector "grep tool is not supported") cases in tools-unsupported.test.ts; no false positive found.
• DISMISSED: no exactOptionalPropertyTypes, bare catch{}, as T, or secret issues found in the diff; typecheck and the structural checks (check:ui-vocabulary, check:report-error, check:error-envelope) all pass.

Targeted tests run (all pass):

• packages/chat: mentions.test.ts (new, 4), tools-unsupported.test.ts, consumer-inference-text.test.ts, test/chat-orchestrator.test.ts, test/parts.test.ts — 77 pass total
• packages/chat-ui: failed-turn-models.test.ts, inference-failure.test.ts, streaming-reply.test.ts, test/failed-turn-strip.test.tsx, test/components.test.tsx — 126 pass total
• bun run typecheck: exit 0