Add TypeAlias ban rule and tinyagent tree hygiene lint

[tunahorse]

Summary

• add ast-grep rule no-typealias-python to ban TypeAlias / TypeAliasType shims
• add rule tests + snapshots and document the policy in docs/rules README files
• add scripts/lint_tinyagent_tree.py to reject __pycache__ and empty/cache-only dirs under tinyagent/
• wire treelint into pre-commit
• simplify proxy non-200 error handling to deterministic status message and replace queue None sentinel with explicit sentinel class

Validation

• sg test --config src/rules/ast/sgconfig.yml
• python3 scripts/lint_tinyagent_tree.py
• pre-commit hooks passed on commit (ruff, mypy, arch checks, vulture, pylint, debtlint, treelint)