- annotate SimpleAgent.__init__ with Any and -> None to improve type
  clarity and mypy compatibility; preserve default model and temp.
- expand AGENTS.md with an Environment section describing local
  aliases and package manager (uv) so contributors understand dev
  assumptions.
- document multi-language support for get_file_outline and list
  supported languages; add example return format for callers.
- add comprehensive tests for codebase explorer tools:
  - language detection mapping for many extensions
  - FileOutlinerTool tests for Python outlines
  - fixtures for temporary directories and outliner setup

These changes improve type safety, developer onboarding docs, and
increase test coverage for multi-language file outlining.

Introduce a DEFAULT_MODEL constant and use it throughout core agents
to avoid hardcoded model names. Add constants.py support for picking
the default model from the environment (OPENAI_MODEL_NAME) with a
fallback to "gpt-4o-mini", and expand the .env.example to document the
OPENAI_MODEL_NAME and common OpenAI-compatible base URLs.

- Add DEFAULT_MODEL in agentic_framework/constants.py and read from
  OPENAI_MODEL_NAME env var.
- Replace hardcoded "gpt-5-nano" with DEFAULT_MODEL in:
  - langgraph_agent.py
  - travel_coordinator_agent.py
  - simple_agent.py
- Update .env.example to document OPENAI_MODEL_NAME and optional
  OPENAI_BASE_URL entries for various providers.

This centralizes model configuration, makes the default model
configurable via environment, and improves maintainability and clarity.

Move dotenv loading out of the CLI module and into the constants module
so environment variables are loaded once at startup before any module reads
them. This prevents subtle ordering issues where modules that import
constants may read env vars before they are loaded.

Update Makefile targets:
- Rename and reorganize check/format/fix/lint targets:
  - Add check target that runs mypy, ruff check and ruff format --check
    (no modifications).
  - Add fix target that runs ruff check --fix and ruff format to apply
    automatic fixes.
  - Remove separate format and lint targets and simplify default target
    list to include fix and check.
- Adjust .PHONY to reflect updated target names.

These changes ensure environment handling is centralized and Makefile
targets are clearer about which commands modify files versus which only
verify state.