feat: add a dependency management graph for agents (#20208)

Relates to https://github.com/coder/internal/issues/1093

This is the first of N pull requests to allow coder script ordering.
It introduces what is for now dead code, but paves the way for various
interfaces that allow coder scripts and other processes to depend on one
another via CLI commands and terraform configurations.

The next step is to add reactivity to the graph, such that changes in
the status of one vertex will propagate and allow other vertices to
change their own statuses.

Concurrency and stress testing yield the following:

CPU Profile:
<img width="1512" height="862" alt="Screenshot 2025-10-17 at 10 38 52"
src="https://github.com/user-attachments/assets/f46cf1a2-a0b2-4c02-81a0-069798108ee5"
/>

Mem Profile:
<img width="1512" height="862" alt="Screenshot 2025-10-17 at 10 38 01"
src="https://github.com/user-attachments/assets/45be1235-fff6-45ba-a50d-db9880377bd0"
/>

Predictably, lock contention and memory allocation are the largest
components of this system under stress. Nothing seems untoward.

.gitignore

@@ -12,6 +12,9 @@ node_modules/
 vendor/
 yarn-error.log
 
+# Test output files
+test-output/
+
 # VSCode settings.
 **/.vscode/*
 # Allow VSCode recommendations and default settings in project root.