Add fail-closed coverage for ask/approval surfaces, attachment ingest, multimodal routing, and child-agent spawn context. Main is still at e2d5f95e; this does not duplicate open coverage PRs #331–#337.

Risky behavior now covered

• Ask questions/options drop raw stream leftovers, extract embedded JSON questions, truncate readable long options, and refuse to render a single surviving option.
• Persisted ask/approval feed lines strip a leading question: label before truncating, omit blank approval commands, and suppress provider debug payloads (raw stream part, response.*, obfuscation).
• Attachment ingest rejects empty, mid-padded, and non-canonical base64 (YR==), plus oversize single/combined/count/upload limits.
• Multimodal MIME mapping sends images only when the model supports them, and keeps audio/video/PDF on Google only.
• Large-media guidance injects only for Google audio/video/PDF when the user asks to extract, and remaps 25MB errors for uploaded multimodal files.
• Child-agent spawn requires a non-blank briefing in brief mode, maps only retired role aliases, and rejects malformed child reports.

Test files added/updated

• test/shared/ask-prompt.test.ts
• test/shared/projection-diagnostics.test.ts
• test/shared/attachments-decode.test.ts
• test/shared/multimodal-mime.test.ts
• test/session/attachment-guidance-policy.test.ts
• test/shared/agent-spawn-context.test.ts

Why these tests materially reduce regression risk

These helpers sit on user-visible ask/approval UI, journal/feed persistence, file ingest, and child-agent context. A silent change could leak raw provider payloads into questions, accept malformed attachments, send binary media to the wrong provider, or spawn a brief-mode agent without a briefing. Dedicated unit tests lock those gates without going through the full session harness.

Validation

• Targeted 26 tests + test/platform-boundary.test.ts green
• bun run check, bun run typecheck, bun run docs:check, and biome check on touched files are clean
• Did not run the full suite on this host (historically truncates late)