fix: remove agents.md (#1540)

## Description

Remove AGENTS.md. Not needed. 


## Type of Change

- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- Removed AGENTS.md

## Testing

<!-- Check what you actually ran, then paste the real command output
below. -->

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [ ] Manual testing performed

### Test Output

```text
NA
```

## Real Behavior Proof

- Environment: NA
- Exact command / steps: NA
- Observed result: NA
- Not tested: NA

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

Add screenshots to help explain your changes.

## Additional Notes

<!-- Mention any N/A checklist items, tradeoffs, follow-ups, or
maintainer context. -->

AGENTS.md

@@ -1,42 +0,0 @@
-<!-- headroom:rtk-instructions -->
-# RTK (Rust Token Killer) - Token-Optimized Commands
-
-When running shell commands, **always prefix with `rtk`**. This reduces context
-usage by 60-90% with zero behavior change. If rtk has no filter for a command,
-it passes through unchanged — so it is always safe to use.
-
-## Key Commands
-```bash
-# Git (59-80% savings)
-rtk git status          rtk git diff            rtk git log
-
-# Files & Search (60-75% savings)
-rtk ls <path>           rtk read <file>         rtk grep <pattern>
-rtk find <pattern>      rtk diff <file>
-
-# Test (90-99% savings) — shows failures only
-rtk pytest tests/       rtk cargo test          rtk test <cmd>
-
-# Build & Lint (80-90% savings) — shows errors only
-rtk tsc                 rtk lint                rtk cargo build
-rtk prettier --check    rtk mypy                rtk ruff check
-
-# Analysis (70-90% savings)
-rtk err <cmd>           rtk log <file>          rtk json <file>
-rtk summary <cmd>       rtk deps                rtk env
-
-# GitHub (26-87% savings)
-rtk gh pr view <n>      rtk gh run list         rtk gh issue list
-
-# Infrastructure (85% savings)
-rtk docker ps           rtk kubectl get         rtk docker logs <c>
-
-# Package managers (70-90% savings)
-rtk pip list            rtk pnpm install        rtk npm run <script>
-```
-
-## Rules
-- In command chains, prefix each segment: `rtk git add . && rtk git commit -m "msg"`
-- For debugging, use raw command without rtk prefix
-- `rtk proxy <cmd>` runs command without filtering but tracks usage
-<!-- /headroom:rtk-instructions -->