Delete deprecated_module_getattr and _renamed_fields #400

deprecated_module_getattr (lite_bootstrap/helpers/warn.py:40-61) has no callers in the package, and BaseConfig._renamed_fields (instruments/base.py:22) is {} on every config. #367 removed the 1.x names and fields that used them. Only their tests keep them alive (tests/test_deprecated_aliases.py, tests/test_renamed_config_fields.py).

Found in the 2.0 audit. Decision: delete them. The next rename brings back whatever it needs, and #350 and #351 show how.

Work

• helpers/warn.py: delete deprecated_module_getattr and NEXT_MAJOR_VERSION. warn_at_caller stays, since the instruments and bootstrappers use it.
• instruments/base.py: delete _renamed_fields and the forwarding loop in BaseConfig.__post_init__. Keep __post_init__ itself, because four subclasses call super().__post_init__() and it ends that chain. Trim its docstring to say only that.
• Drop the UNSET and UnsetType imports from instruments/base.py. fastapi_bootstrapper.py still uses both, so types.py stays.
• Delete tests/test_deprecated_aliases.py and tests/test_renamed_config_fields.py.
• AGENTS.md, "Versioning": the rename bullet keeps the rule (the old name works with a DeprecationWarning for at least one minor, and the next major deletes it) but stops naming the two helpers. Point to feat: warn on deprecated aliases ahead of their removal in 2.0 #350 and feat: rename inconsistent config fields, keeping the old names as deprecated aliases #351 for how the last rename did it.

Breaking changes

None for users. Both helpers are internal: lite_bootstrap.helpers.warn isn't re-exported, and _renamed_fields is private.