docs: add AGENTS.md contributor guide for AI agents

[ethanndickson]

Adds an AGENTS.md contributor guide at the repo root to give AI coding agents the high-signal context they need to work in this provider safely.

The document captures the provider's framework idioms (terraform-plugin-framework, not SDKv2), the resource file layout, essential make commands, and — most importantly — the project-specific patterns and anti-patterns distilled from past fixes (e.g. handling unknown values at plan/validate time, refreshing cached entitlements after a license apply, not stripping cty sensitivity marks in plan modifiers, and avoiding defer inside retry loops). Each anti-pattern references the PR/issue it was learned from.

Contents were verified against the current tree: Makefile targets, the util.go helpers, provider.go entitlement plumbing, the CI Terraform 1.0–1.9 acceptance matrix, and the golangci paralleltest config.

[ethanndickson]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: add experimental coderd_agents_model resource #371
• docs: add AGENTS.md contributor guide for AI agents #369 👈 (View in Graphite)
• feat: add experimental coderd_ai_provider resource #368
• ci: run acceptance tests against an external Postgres sidecar #370
• main

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