perf(acp): bound automatic conversation context

[bvgeertagent]

Summary

• cap automatically fetched thread/DM content at 32 KiB by default
• preserve the thread root and newest messages while truncating on UTF-8 boundaries
• expose prompt-build duration and context size/truncation in tracing and observer telemetry
• add BUZZ_ACP_CONTEXT_BYTE_LIMIT / --context-byte-limit with 0 as the legacy unlimited opt-out

At the existing 12-message window and 64 KiB event-content ceiling, automatic context content is now bounded from 786,432 bytes to 32,768 bytes (24x smaller) before prompt framing. Full history remains available through the existing Buzz CLI retrieval hints.

Validation

• cargo fmt --all --check
• cargo check -p buzz-acp
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo test -p buzz-acp context_byte (6 passed)
• full package comparison on Windows: baseline 800 passed / 28 failed; branch 806 passed / 28 failed. The same 28 existing failures are POSIX process-fixture incompatibilities under Git Bash/Windows paths.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 00e61eafa917d296104006576b7a2ddbfd58bb5a...12d709ca5b2efa725a986f0d3f28a4c80799e25b.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review 12d709ca5b2efa725a986f0d3f28a4c80799e25b to authorize a new review.
Any previous review applies only to its recorded range.