docs: add CONTRIBUTING, AGENTS, and PR template

[bharvey88]

Contributor docs so PRs (human or agent) hit the repo's gotchas before CI does: CONTRIBUTING.md (local dev, tests, adding a device, deploy rules), AGENTS.md (same facts condensed for coding agents), and a minimal PR checklist template.

Prompted by #2 arriving before any of this existed.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Added contributor guidance and a new pull request template.
  • Documented how to run the site locally, test changes, and validate device registry updates.
  • Clarified the process for adding device entries and images, plus key rules to avoid deployment issues.

[coderabbitai]

Walkthrough

This PR adds three documentation files: a GitHub PR template with a checklist, an AGENTS.md file describing repository conventions and workflow for contributors, and a CONTRIBUTING.md file documenting local setup, testing, and device-contribution processes. No code changes are included.

Changes

Contributor Documentation

Layer / File(s)	Summary
PR template and checklist .github/PULL_REQUEST_TEMPLATE.md	Adds a "What does this change?" prompt and a checklist covering Playwright tests, pages.yml asset updates, avoiding new build/CDN dependencies, and running the registry validation script.
Contributor and workflow guidance AGENTS.md	Documents the static no-build setup, serve/test/validation commands, devices.json as source of truth, Pages deployment file requirements, vendored esp-web-tools restrictions, and security/stale-write patterns for js/views/device.js.
Contribution setup and process docs CONTRIBUTING.md	Documents local run/test instructions, CI validation steps, the device-addition workflow via devices.json and scripts/fetch_images.py, and rules preventing CI/deployment failures.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Poem

Three new scrolls upon the shelf,
AGENTS, CONTRIBUTING, template itself,
This bunny hops through docs with glee,
No code to break, just words set free,
Thump thump — review complete! 🐇📜

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main documentation additions: CONTRIBUTING.md, AGENTS.md, and the PR template.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch contributor-docs

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@CONTRIBUTING.md`:
- Around line 5-7: Add a shell language tag to both fenced command blocks in the
contributing docs so markdownlint stops flagging them. Update the markdown
fences around the commands shown near the existing python and npm test commands,
and make sure both code blocks use the same explicit bash tag.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 190b088d-fb9f-44d2-a022-3ef94bbfe8c2

📥 Commits

Reviewing files that changed from the base of the PR and between 6327e4f and 30a3adc.

📒 Files selected for processing (3)

• .github/PULL_REQUEST_TEMPLATE.md
• AGENTS.md
• CONTRIBUTING.md

[coderabbitai]

📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Add a language tag to both fenced command blocks.

markdownlint already flags these blocks, so the docs will stay noisy until the fences declare the shell language.

🔧 Minimal fix

-```
+```bash
 python -m http.server 8123

```diff
-```
+```bash
 cd tests
 npm ci
 npx playwright test

</details>


Also applies to: 13-17

<details>
<summary>🧰 Tools</summary>

<details>
<summary>🪛 markdownlint-cli2 (0.22.1)</summary>

[warning] 5-5: Fenced code blocks should have a language specified

(MD040, fenced-code-language)

</details>

</details>

<details>
<summary>🤖 Prompt for AI Agents</summary>

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In @CONTRIBUTING.md around lines 5 - 7, Add a shell language tag to both fenced
command blocks in the contributing docs so markdownlint stops flagging them.
Update the markdown fences around the commands shown near the existing python
and npm test commands, and make sure both code blocks use the same explicit bash
tag.

</details>

<!-- fingerprinting:phantom:triton:quartz -->

<!-- cr-indicator-types:potential_issue -->

<!-- cr-comment:v1:be9e32548d20740d5c12954d -->

_Source: Linters/SAST tools_

<!-- This is an auto-generated comment by CodeRabbit -->