warn: emit stderr warning on RTK_DISABLED=1 (#508)

[ousamabenyounes]

Summary

Fixes #508 — AI agents overuse RTK_DISABLED=1 after encountering a single filtering issue, silently killing token savings for the entire session.

Real impact observed: 801 commands in a session, 243 (30%) used RTK_DISABLED=1 unnecessarily. Savings dropped from ~65% to 48%.

Fix: When the rewrite hook detects RTK_DISABLED=1, emit a warning on stderr:

[rtk] RTK_DISABLED=1 detected — skipping filter for this command. Remove RTK_DISABLED=1 to restore token savings.

The agent sees this warning in real-time and learns to stop overusing the bypass.

Why this works

• Warning is on stderr (not stdout) — no impact on command output or piping
• The bypass still works exactly as before — no behavior change
• Agents (Claude, Gemini) read stderr and adjust behavior
• A single warning per command is enough to break the pattern

Before/After

# BEFORE: agent silently bypasses RTK on 30% of commands
RTK_DISABLED=1 git status    # no output, no warning → agent keeps doing it

# AFTER: agent sees the cost
RTK_DISABLED=1 git status
# stderr: [rtk] RTK_DISABLED=1 detected — skipping filter for this command.
#         Remove RTK_DISABLED=1 to restore token savings.
# → agent stops overusing it

Test plan

• 3 existing RTK_DISABLED tests pass (return value unchanged)
• Full suite: 764 passed, 0 failed
• cargo fmt && cargo clippy && cargo test all green
• validate-docs.sh passes

Generated with Claude Code