fix(workflows): match overlay file extensions case-insensitively

[jawwad-ali]

Problem

ProjectOverlaySource.collect matches the extension verbatim:

if not path.is_file() or path.suffix not in (".yml", ".yaml"):
    continue

So a hand-placed overlay named <id>.YML or <id>.Yaml is skipped and never applied — and nothing is reported to say the file was ignored. Overlay files are explicitly hand-authored (docs/reference/workflows.md documents the format and tells users to write them), so the extension casing is the author's choice, not something the tool generated.

Reproduction on current main (c173bf1)

Three overlays in one directory, differing only in extension case:

files on disk: ['Mixed.Yaml', 'UPPER.YML', 'lower.yml']
COLLECTED    : ['lower']
SKIPPED      : ['mixed', 'upper']

The author gets no error and no warning — the overlay simply has no effect.

Why this is the odd one out

Every other YAML discovery path in the package already lowercases before matching:

location	form
workflows/engine.py:941	path.suffix.lower() in (".yml", ".yaml")
workflows/_commands.py:1325	source_path.suffix.lower() in (...)
workflows/_commands.py:1894	dev_path.suffix.lower() in (...)
workflows/_commands.py:2128	source_path.suffix.lower() in (...)
overlays/layer_sources.py:150	path.suffix in (...) ← this PR

Fix

if not path.is_file() or path.suffix.lower() not in (".yml", ".yaml"):

Scope note

overlays/_commands.py:115 (_find_overlay_file) carries the identical case-sensitive pattern. I have deliberately not touched it here: an open PR of mine (#4141) already modifies that file, and combining the two would create a needless conflict. Happy to follow up with it once #4141 lands, or to fold it in here if you would rather see both together.

Verification

• Fail-before / pass-after: 4 new-vs-baseline failures with the source reverted to upstream/main → passing with the fix.
• Parametrized over .YML, .Yaml, .YAML, .Yml.
• A companion test asserts broadening the case did not broaden which extensions are accepted — notes.txt, backup.yml.bak, README.md and data.json are still skipped.
• Scoped regression over tests/workflows: no new failures vs a clean-main self-baseline (10 pre-existing, Windows symlink-privilege).
• uvx ruff@0.15.0 check src tests → clean

Behaviour change, disclosed: files previously skipped now get read. If a project already holds a .YML/.YAML file in an overlay directory that is not a valid overlay manifest, resolution will now report it rather than ignoring it — which is the intended outcome, but it is a change for such a project.

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Written with assistance from Claude Code. Bug found, reproduced, and verified by me on current main.

🤖 Generated with Claude Code