Add configurable start timeout

[Haihan-Jiang]

Fixes #108

This adds a --start-timeout SECONDS option for composer-dev start and composer-dev restart. The default behavior remains 300 seconds, and --start-timeout 0 disables the timeout for cases where dependency installation needs to run longer.

Changes:

• pass the configured timeout from the CLI into environment startup
• apply the same timeout to database and main environment readiness waits
• include the effective timeout in the timeout error message
• cover default, custom, disabled, and invalid timeout cases in unit tests

Tested:

• python -m pytest tests/unit/test_cli.py::TestStartRestartCommand tests/unit/test_environment.py::TestWaitForStart -q
• python -m pytest tests/unit -q
• python -m black --check composer_local_dev tests/unit

[Haihan-Jiang]

This is ready for review. The change adds a configurable start timeout for local Composer startup flows, and the visible checks are green.