fix: handle expression arg that matches a filename

[sjh9714]

Summary

Fixes expression arguments that happen to match an existing filename.

What changed

• Added processArgs() regression coverage for .version when a same-named file exists and an input file or stdin follows it.
• Updated first-argument detection so expression-looking dot args followed by input are treated as expressions.
• Preserved single-file usage, .yq expression files, and explicit path-qualified file args such as ./.version.

Why

Before this change, processArgs() skipped expression detection when the first arg existed as a file, so yq '.version' data.yml could read .version as an input file instead of evaluating .version against data.yml.

After this change, .version data.yml and .version - are parsed as expression plus input, while users can still force file interpretation with ./.version.

Testing

• go test ./cmd -run TestProcessArgs -count=1 failed before the production fix with .version left in args instead of becoming the expression.
• go test ./cmd -run TestProcessArgs -count=1
• go test ./cmd -count=1
• go test ./...
• bash ./scripts/devtools.sh
• make local test
• git diff --check

Fixes #2477

Note: I used Codex to help inspect the code path and draft the regression test, but I reviewed the final diff and ran the listed verification commands locally.