fix(context): warn when a context file is truncated instead of dropping it silently

[BKStock]

Stacked on #72448 (shares tests/agent/test_subdirectory_hints.py). Review that one first; this diff is the two logger.warning calls plus their tests.

Problem

Both context-loading paths shed content without telling anyone who can act on it.

_truncate_content() inserts an in-band marker:

[...truncated AGENTS.md: kept 14000+4000 of 52488 chars. Use file tools to read the full file.]

That string is addressed to the model. The person who wrote the file gets nothing — no log line, no startup notice, no non-zero exit. A repo whose AGENTS.md outgrows CONTEXT_FILE_MAX_CHARS keeps booting cleanly while the middle of the file stops reaching the model.

This repo's own AGENTS.md is the case in point. At 52,488 chars against a 20,000 cap with a 70/20 head-tail split, 66% was dropped on every single session. The discarded middle was not filler — Plugins, Skills, Toolsets, Delegation, Curator, Cron, Kanban, Important Policies, Profiles, and Known Pitfalls all sat inside it. The prompt-caching rules and the entire pitfalls list were never actually loaded, and nothing anywhere said so.

The subdirectory-hint loader has the same shape at its own 8,000-char limit.

Change

Both truncation points log a WARNING naming the file, its size, the limit, how much was dropped, and what to do about it:

WARNING agent.prompt_builder: Context file AGENTS.md is 52488 chars (limit 20000)
— dropping 34488 chars (66%) from the middle. Split the overflow into separate
docs and reference them, or the omitted sections will never reach the model.

Files under the limit are returned untouched and stay silent, so a warning firing means content was genuinely lost — no ambient noise to tune out.

Tests

4 added. Each path: warns when oversized, silent when within budget. 159 passed, 1 skipped.

Run directly with python -m pytest; the wrapper's venv probe is fixed separately in #72447.

[teknium1]

Thanks for identifying the silent subdirectory-context truncation path. The remaining subdirectory warning is still useful on current main.

Problems

• The startup-context half is already superseded: agent/prompt_builder.py:1965-1971 logs and records truncation warnings, and agent/system_prompt.py:568-571 emits them through the user-visible status channel. The PR has diverged substantially, so that hunk should not be reapplied over the current dynamic-cap implementation.

Suggested changes

• Salvage only the missing warning around agent/subdirectory_hints.py:235-239.
• Make the new subdirectory test assert the warning’s file/path and limit fields as well as its presence.

Automated hermes-sweeper review.