docs: add Corridor agent instructions (#779)

## Summary

Add a repository-wide `AGENTS.md` with an explicit `<corridor>` block
requiring agents to plan first and run Corridor `analyzePlan` before
generating or modifying code.

The tags are balanced so Corridor-specific guidance remains scoped.

## Validation

- `git diff --check`
- Corridor tag-balance assertion

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Co-authored-by: langsmith-fleet[bot] <langsmith-fleet[bot]@users.noreply.github.com>