Refactor boolean handling to centralised utility

[Chaffelson]

extract parse_bool() utility for CLI boolean parameter handling

Addresses #397 - Extract boolean parsing logic from getenv_bool() into a reusable parse_bool() utility function.

Changes:

• Add parse_bool() to nipyapi/utils.py for handling CLI boolean parameters where fire passes --flag=false as string 'false'
• Refactor getenv_bool() to use parse_bool() internally
• Remove duplicate _env_bool() from ci/cleanup.py and ci/stop_flow.py
• Update ci/configure_inherited_params.py to use parse_bool()
• Update docs/contributing.rst CLI Boolean Parameters section with examples
• Add comprehensive TestParseBool test class in test_utils.py
• Remove obsolete TestEnvBool from test_ci.py (covered by new tests)

The parse_bool() function handles:

• String values: 'false', '0', 'no', 'off', 'n' -> False
• String values: 'true', '1', 'yes', 'on', 'y' -> True
• Bool passthrough: True/False unchanged
• None handling with default parameter
• Other types: uses Python truthiness

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.46%. Comparing base (0716aee) to head (d268af4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
nipyapi/ci/cleanup.py	60.00%	4 Missing ⚠️
nipyapi/ci/configure_inherited_params.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #401   +/-   ##
=======================================
  Coverage   76.45%   76.46%           
=======================================
  Files          41       41           
  Lines        4732     4734    +2     
=======================================
+ Hits         3618     3620    +2     
  Misses       1114     1114           

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[ottobackwards]

lgtm!