docs(action): fix inaccuracies and add missing metadata to action.yml

[kranthipoturaju]

Summary

Corrects and completes the metadata in action.yml to accurately reflect the runtime behavior of the action. No behavioral changes — documentation only.

Changes

• persist-credentials description: Updated to reflect the v6 change where credentials are stored in a file under $RUNNER_TEMP (not written directly
  into .git/config), and that the post-job step removes that file.
• filter description: Replaced the incorrect "Overrides sparse-checkout if set" with an accurate explanation — when filter and sparse-checkout are both set, the provided filter is used for git fetch instead of the automatic blob:none; sparse checkout itself still runs.
• filter / sparse-checkout defaults: Removed default: null. YAML null is not a meaningful default for optional string inputs; omitting the key correctly signals no default value.
• path default: Added default: '.' to document the code-level fallback (core.getInput('path') || '.').
• required: false: Added to all optional inputs that had no explicit required field (ref, ssh-key, ssh-known-hosts, path, filter, sparse-checkout, github-server-url).
• ref output description: Corrected — when the ref input is a full 40- or 64-character SHA, the code sets sourceSettings.ref = '', so this output is empty for SHA-based checkouts.
• commit output description: Added caveat that this output is only emitted in the git code path (git log --format=%H); it is never set when the action falls back to the GitHub REST API downloader.

Motivation

Several descriptions in action.yml were stale (written before the v6 credential storage change), inaccurate (filter description, output descriptions), or incomplete (missing defaults and required flags). These inconsistencies could mislead users relying on the marketplace UI, tooling that parses action.yml, or anyone reading the source to understand the contract.

Impact

• Users: No behavioral change. All input/output names and defaults remain backward compatible.
• Tooling: Tools that parse action.yml (e.g., IDE extensions, linters, action documentation generators) will now see correct required and default values.
• path default: Consumers setting path: '.' explicitly may notice it is now the documented default — no functional difference.

Testing

Documentation-only change. Validated by cross-referencing each modified field against the implementation in:

File	Verified
src/input-helper.ts	Input defaults and required semantics
src/git-source-provider.ts	commit output only emitted in git path
src/main.ts	ref output and SHA-based ref handling
src/git-auth-helper.ts	persist-credentials / $RUNNER_TEMP behavior

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the GitHub Action metadata to better document optional inputs/outputs and clarify behavior for credential persistence and checkout options.

Changes:

• Mark several inputs as explicitly optional (required: false) and add a default for path.
• Expand/clarify descriptions for persist-credentials, filter, and outputs (ref, commit).
• Remove default: null from inputs where absence should indicate “unset”.

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