fix: skip ignored directories early during file walk

[CreatorHead]

Previously, ignore patterns were only checked against files — the walk still descended into every directory unconditionally. This meant a directory like .venv with thousands of files was fully traversed before each file was individually filtered out, causing significant slowdowns.

Now, when a directory itself matches an ignore pattern, filepath.SkipDir is returned immediately, pruning the entire subtree in one step regardless of how many files are inside.

🛠️ Description

What was broken:

The file walk in both addlicense/main.go and cmd/headers.go only applied ignore pattern checks to files. When the walk encountered a directory, it always returned nil and descended into it unconditionally. This meant a directory like .venv with thousands of Python files was fully traversed — visiting every file, checking each one against the pattern, and then discarding it. The cost scaled linearly with the number of files in the ignored directory.

What changed:

Added a directory-level ignore check in both walk functions. When a directory path matches an ignore pattern, filepath.SkipDir is returned immediately, telling the OS not to read the directory's children at all. The path != start / path != "." guard ensures the root directory itself is never accidentally skipped.

🔗 External Links

👍 Definition of Done

• New functionality works? — verified across 17 test scenarios including directory skipping, nested patterns, and all pre-existing header behaviours
• Tests added? — all existing tests pass; manual regression suite run below

Test Results

#	Scenario	Expected	Result
A	All headers correct, LICENSE up to date	No changes	✅ Clean
B	Source file missing header, LICENSE already at current year	Header added, LICENSE untouched	✅ Pass
C	Source file missing header, LICENSE behind (2025)	Header added + LICENSE bumped	✅ Pass
D	Source file year outdated (2025), LICENSE behind	Both year-bumped to current year	✅ Pass
E	--plan mode with missing header	Shows what would change, writes nothing	✅ Pass
F	--plan mode with everything correct	No output, writes nothing	✅ Clean
G	No LICENSE file in repo	Runs cleanly, no crash	✅ Pass
H	Mixed — correct, missing, outdated in same repo	Only affected files changed, correct file untouched	✅ Pass
I	ignore_year1=true, single-year headers already correct	No changes	✅ Clean
J	Idempotency — run once then run again immediately	Second run always produces zero changes	✅ Pass
K	Existing header year outdated, no missing headers (step 1 only)	Year bumped in source + LICENSE	✅ Pass
L	Copyright holder mismatch (e.g. HashiCorp → Acme)	Holder updated, LICENSE untouched if already current year	✅ Pass
M	.venv in header_ignore, 100 files inside	Files inside .venv not touched	✅ Pass
N	Multiple ignored dirs (vendor/** + **/node_modules/**)	Neither directory traversed	✅ Pass
O	Ignored dir alongside a source file missing a header	Source file updated, ignored dir untouched	✅ Pass
P	Nested ignored dir (**/vendor/** inside a subdirectory)	Nested dir skipped correctly	✅ Pass
Q	200 files in .venv — verify single skip log entry	skipping directory: .venv appears exactly once	✅ Pass

🤔 Can be merged upon approval?

✅

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request. — simple git revert; no schema, data, or config changes involved.
• If applicable, I've documented the impact of any changes to security controls. — no security controls affected; this is a performance fix in directory traversal logic only.