Triage: Surface guarded refusals for autonomous triggers

[time-attack]

Tracking the change proposed in #664 (by @wsr3005).

We're moving this from a code PR to a tracked issue under CONTRIBUTING.md: feature ideas belong in adrs/ as a short human-written note and bugs as an issue, and we implement the underlying code from our side. Capturing the original proposal here so it isn't lost.

Original PR: #664
Reported by: @wsr3005

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Summary

• classify budget, rate-limit, and security-quarantine refusals on TurnResult
• deliver concise, idempotent budget and rate-limit notices for automated poll surfaces
• keep security-quarantine details silent for autonomous turns and redacted across Slack refusal paths
• preserve existing error-notice behavior without double-delivering guarded refusals

Verification

• affected tests: 218 passed
• npm run typecheck
• ESLint and Oxlint on affected files
• Prettier and git diff --check
• synthetic rendering of the automated notices; no live Slack workspace was used

Screenshot

The screenshot uses synthetic data. Security-quarantine details remain silent for autonomous turns.

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If we ship a fix or feature from this, credit @wsr3005 as co-author.