fix: LICENSE file no longer updated on every run when no headers changed

[CreatorHead]

anyFileUpdated was unconditionally set to true in non-plan mode due to a Repetitive condition, causing the LICENSE year to be bumped on every run even when all source file headers were already correct. This triggered buggy pre-commit hook failures.

Fix: teach addlicense.Run to return whether it actually wrote any files, and use that signal instead of the always-true condition.

🛠️ Description

What was broken:

The anyFileUpdated flag in cmd/headers.go was supposed to be true only when at least one source file was actually modified. The condition guarding it was a tautology — the second branch (!plan && runErr == nil) is always true in normal mode, so anyFileUpdated was set on every successful run regardless of whether any file was touched. This caused the LICENSE file's copyright year to be bumped unconditionally, producing a spurious git diff and causing pre-commit hooks to fail with "files were modified by this hook" even when no header work was needed.

What changed:

• addlicense.processFile now returns (bool, error) — the bool is true when the file was actually written to
• addlicense.Run now returns (bool, error) — uses an atomic.Bool to aggregate results across parallel goroutines
• The always-true condition in cmd/headers.go is replaced with the actual signal from Run

Before:

if runErr != nil || (!plan && runErr == nil) {
    anyFileUpdated = true
}

After:

filesAdded, runErr := addlicense.Run(...)
if runErr != nil || filesAdded {
    anyFileUpdated = true
}

🔗 External Links

👍 Definition of Done
New functionality works? — verified: repo with all correct headers produces zero changes and clean git status
Tests added? — existing test suite updated to match new return signature; all packages 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 or data changes involved.
If applicable, I've documented the impact of any changes to security controls. — no security controls affected; bug fix in copyright year tracking logic only.

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