docs: add agent guidance and contributor setup

[V3RON]

Summary

This updates the repo guidance so contributors and coding agents can make smaller, more consistent changes with less unnecessary exploration. It adds a repo-specific AGENTS.md and a concise CONTRIBUTING.md that explain how to work on agent-cdp, how CLI output should stay token-efficient, and how PRs should describe user impact, compatibility, risks, and manual testing.

Backward Compatibility

There are no command, flag, output, artifact, or daemon behavior changes in this PR. The changes are documentation-only, so existing user workflows and scripts remain compatible.

Risks

• Low risk overall because this PR only adds and updates docs.
• The main regression risk is process drift if future CLI changes are made without keeping AGENTS.md, CONTRIBUTING.md, src/cli.ts, and packages/agent-cdp/skills/core.md aligned.
• The new guidance intentionally pushes contributors toward concise default CLI output, so future doc follow-through matters for Chrome and React Native workflows equally.

Manual testing

Scenario: validate that a new contributor or coding agent can discover the intended workflow without reading unrelated files.

Setup assumptions:

• The branch is checked out locally.
• Dependencies are already installed.

Steps:

1. Open AGENTS.md and verify it contains the sections First Minute, Scope, Code Changes, Toolchain, Cheap Exploration, Command Family Lookup, Pull Requests, and Priority Order.
2. Verify AGENTS.md includes guidance to keep CLI output token-efficient, avoid JSON as the default output format, preserve daemon/session architecture boundaries, and update src/cli.ts plus packages/agent-cdp/skills/core.md when CLI workflows change.
3. Open CONTRIBUTING.md and verify it explains the repo-specific first-pass workflow, root validation commands, built CLI workflow, and the daemon/target/manual loop without duplicating obvious generic setup advice.
4. Confirm there are no source-code changes outside the two new docs files.

Expected outcome:

• A contributor can understand how to approach agent-cdp changes, where to look first, how to validate changes, and how to write a PR without needing extra project-specific tribal knowledge.