Use generative approval copy and plain-text fallback for channel approvals

[noanflaherty]

Summary

• route approval/guardian user-facing copy through composeApprovalMessageGenerative across channel approval routes
• add best-effort rich-to-text fallback for approval prompts so Telegram/button failures degrade to actionable plain text
• send non-rich channel approvals/reminders as plain text without approval metadata payloads
• update approval route tests to validate text-fallback semantics instead of rich-delivery hard failure

Validation

• cd assistant && bun test src/__tests__/channel-approval-routes.test.ts src/__tests__/approval-message-composer.test.ts src/__tests__/approval-hardcoded-copy-guard.test.ts
• cd assistant && bunx tsc --noEmit (fails due pre-existing unrelated errors in src/__tests__/sms-messaging-provider.test.ts and src/daemon/handlers/config.ts)

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 5 additional findings.

[chatgpt-codex-connector]

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[noanflaherty]

Addressed in #7518