docs: slim AGENTS.md, drop the agent-docs check, simplify the PR template

[datlechin]

Summary

• AGENTS.md 7.9 KB to 2.3 KB, apple/AGENTS.md 11.6 KB to 6.8 KB: an Apple task loads 9.1 KB instead of 19.5 KB.
• Cut lines a tool enforces, general advice, the doc budgets, the verdict bullets verification-apple.md already holds, and the architecture the theme, editor and storage rules already hold.
• Added the agent rights line: --auto --squash on green, release/* and tags only when the maintainer says to release, --admin only on the maintainer's word.
• CI section points to ci.md and the rules API instead of listing the checks.
• Area rules table replaced with one line: read each rule whose paths: header matches the file.
• Deleted scripts/check-doc-symbols.sh, the verify.sh agent-docs step and every pointer to them.
• PR template: title comment, Summary, Test plan, Fixes #.
• apple/CONTRIBUTING.md: dropped the PR checklist and the Automation Mode row. linux/CONTRIBUTING.md: dropped the second GTK env paragraph and the restated clippy denies.
• fix-issue: pointer fixes only, for the removed table, step and template scopes.

Test plan

• python3 -m unittest scripts/ci/test_detect_changed_paths.py: 24 pass.
• verify.sh agent-docs now exits 3 as an unknown step.
• Every path and section the edited docs name exists; .claude/rules paths: headers unchanged.
• scripts/check-banned-words.sh --staged passes.