Document the required CI checks and the job-rename hazard

[CarsonDavis]

Documents the merge-gating rules that #201 adds to the require-pr ruleset, in the place a contributor will look before touching workflows.

• New "Required CI Checks" subsection in AGENTS.md's Git Workflow section: PRs into development/main/production must pass unit, config-generation, e2e (full), and e2e (lean) (all from playwright-tests.yml) on top of the existing 1-approval requirement; "not green" includes still-running and never-reported checks.
• The rename hazard, stated plainly: required check names match as plain strings, so renaming a job in playwright-tests.yml silently blocks every PR until an admin updates the ruleset — job renames there are ruleset-affecting changes.
• Bypass policy recorded: nobody, including admins; an emergency override is an audited edit to the ruleset itself.

The companion ruleset change (adding the required_status_checks rule to ruleset require-pr, id 12749875) is a live-repo admin action applied separately once this PR is reviewed; the payload has been verified against the live config and the check contexts on the latest merged PR head.

Closes #201