feat(acp): include thread root kind in context

[malkovitc]

Summary

• retain the fetched thread root event kind in ACP conversation context
• expose Thread root kind in the agent's [Context] block for Forum and stream threads
• include Forum comments and stream diffs in thread reply/history/count queries
• preserve root-kind metadata when delivery dedup filters prior context

This is the minimal context slice from upstream block#3596. Setup nudges, dead-letter publication, CLI-wide reply classification, and broad filter refactors are intentionally deferred as separate work.

Verification

• cargo test -p buzz-acp --lib — 819 passed
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --all -- --check
• file-size ratchet and git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve root kind when the context delta is empty

On a subsequent turn in the same thread, the fetched root and prior replies are commonly all in delivered, while the new reply is in triggering; filtering therefore leaves no messages and the then_some discards the entire context, including the newly captured root_kind. The resulting [Context] omits Thread root kind precisely after history has already been delivered, so this metadata should be carried independently of whether any context messages remain.

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