Deduplicate ACP thread prompt context

[salman1993]

Why

This is a small isolated follow-up to #6503: remove two proven duplicate prompt fragments without adopting its broader framing experiment or changing Buzz-native routing and identity behavior.

What

• Remove parsed parent=<id> only when it exactly duplicates parsed root=<id>; retain root=<id>, nested parents where parent differs from root, raw event tags, event IDs, author identity, mention fields and rules, and the explicit --reply-to instruction.
• Remove the full-history retrieval hint only when complete, untruncated conversation or thread context is included. Thread context must include its root and cover every event rendered in the batch; mixed-thread batches and root-missing windows retain retrieval guidance. Context that is absent, previously delivered, or truncated also retains retrieval guidance, and the context header, count, and messages remain unchanged.

Risk Assessment

Low — the change is confined to Buzz ACP prompt rendering and preserves all routing, identity, event, and retrieval information outside the two exact duplicate cases.

Testing

• cargo test -p buzz-acp — 804 unit tests and 9 lifecycle integration tests passed on the clean rerun
• Focused mixed-thread, same-thread, and missing-root prompt regressions passed
• The timing-sensitive keepalive_resets_idle_past_deadline test passed in isolation after one unrelated full-suite flake, then passed in the clean full rerun
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --all -- --check
• just file-size-check
• Repository pre-push gate at 1b88793574e74e22f00a1fd19bfe9aa8e86150fa — file-size, branch, Rust unit-test, and Desktop Tauri lanes passed
• git diff --check

Generated with Codex

[salman1993]

🤖 Re-review at exact head 1b88793574e74e22f00a1fd19bfe9aa8e86150fa: approved — no actionable findings.

The prior mixed-thread and missing-root blockers are fixed. The retrieval hint is now omitted only when the fetched window is untruncated, includes the thread root where applicable, covers every rendered batch/cancelled event, and contains no previously delivered events. Mixed-thread, reply-only, truncated, absent, and delivery-delta cases keep the escape hatch. parent is removed only when it equals root; nested parents, raw tags, event IDs, identity/mentions, and reply routing remain.

Validation:

• Independently verified by FastValidator: full cargo test -p buzz-acp (804 unit + 9 lifecycle passed), clippy, and fmt at this SHA.
• Independently rechecked here: the four focused regressions pass; git diff --check is clean.
• Live-local pass against a fresh database and relay built at this SHA: a real thread root + prior reply + live reply reached a scripted ACP through session/prompt; the prompt retained root/history/raw tags/routing, omitted duplicate parent, and omitted the retrieval hint for that complete same-thread turn.
• CI's Unit Tests job is red only because buzz-voice cannot link sherpa-onnx-c-api; the same failure is present on a later unrelated main run (32783072355). All other PR checks are green.

Product fit: this remains the narrow deletion-oriented follow-up to closed #6503 and aligns with VISION_AGENT.md's minimal/auditable boundary without adopting the broader envelope framing.

Quality bar: minimalness 9/10, elegance 9/10, correctness 9/10.