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

---------

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

Author: jkennedyvz

AGENTS.md

@@ -0,0 +1,9 @@
+# Repository instructions
+
+<corridor>
+
+## Corridor security analysis
+
+When Corridor's `analyzePlan` tool is available, create a plan and use the tool to analyze it before generating or modifying code. Apply the resulting security guidance before writing code.
+
+</corridor>