Fail explicit config writes when AF_CONFIG is non-regular

[RitwijParmar]

Summary

• Adds a ConfigManager.require_writable() guard that preserves neutralized-config read/no-op behavior while giving explicit config mutations a clear failure path.
• Wires the guard through af instance add/use/delete/reset, instance discovery write preflight, and af telemetry enable/disable.
• Adds CLI and layered-config tests for directory-backed AF_CONFIG as the platform-neutral stand-in for /dev/null.

Fixes #194.

Verification

• /Users/ritwij/Library/Python/3.9/bin/uv run pytest tests/test_cli_instances.py tests/test_cli_config_writes.py tests/test_layered.py tests/test_config.py (137 passed)
• /Users/ritwij/Library/Python/3.9/bin/uv run pytest tests --ignore=tests/integration (574 passed)
• /Users/ritwij/Library/Python/3.9/bin/uv run ruff check src tests/test_cli_instances.py tests/test_cli_config_writes.py tests/test_layered.py
• /Users/ritwij/Library/Python/3.9/bin/uv run ruff format --check src tests/test_cli_instances.py tests/test_cli_config_writes.py tests/test_layered.py

Full pytest was also attempted; only tests/integration/test_api_endpoints.py failed because no Airflow server was listening on localhost:8080 in my local environment.