docs: tighten agent and pull request guidance

[Azganoth]

Summary

Clarify repository guidance where recent agent work exposed ambiguity:

• Route workflow, documentation, pull request, project-management, and release work directly to their authoritative sections.
• Reserve code comments for non-obvious rationale, constraints, and consequences instead of narrating implementation.
• Allow accepted behavior changes to update or replace stale tests, fixtures, snapshots, and specifications while keeping enforcement guardrails explicit.
• Prefer equivalent Tailwind utilities through @apply in CSS files, with clear exceptions for behavior Tailwind cannot express precisely.
• Standardize pull request verification around focused behavioral evidence, manual scenarios, and relevant omissions without reproducing CI output.

Related Issue

Not applicable. Focused repository-guidance update based on recurring contribution and agent-work patterns.

Verification

pnpm exec oxfmt --check AGENTS.md src/AGENTS.md .github/pull_request_template.md and git diff --check.

Confirmed that every newly linked documentation path and section heading resolves.

Documentation-only; no executable configuration or application behavior changed.