fix(agent): report truncated project hints

[kiranmagic7]

Summary

load_hint_files_impl silently stopped at the first AGENTS.md that crossed the 128 KiB project-hints budget. It also appended the inter-file separator before checking the remaining space, so an exact-budget parent followed by a deeper file returned 131,074 bytes while dropping the deeper file without any signal.

This keeps the combined output within 128 KiB, emits a structured warning naming the file that crossed the limit, and reserves room inside the same budget for an in-band truncation marker. File ordering and UTF-8 boundary handling are unchanged.

Related issue

Follow-up to #4163 and #4167. The discussion on #4163 identifies the AGENTS.md chain as a separate silent-loss path, and #4167 intentionally limits its fix to load_skill.

Duplicate search: no focused issue or PR was found for load_hint_files_impl or MAX_HINTS_BYTES. #3732 touches home-directory resolution in the same file, while #3262 is a broad draft refactor.

Testing

• cargo test -p buzz-agent load_hint_files_reports_ -- --nocapture
• cargo test -p buzz-agent
• cargo clippy -p buzz-agent --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• just ci

All passed on macOS arm64. No UI change; screenshots are not applicable.