Add version-file input to read the Packer version from a .tool-versions file

[somaz94]

This adds an optional version-file input so the Packer version can be read from a .tool-versions file (the format shared by asdf and mise), rather than hard-coding the version in the workflow. This lets a repository keep a single source of truth for its Packer version across local tooling and CI.

The input is fully opt-in and backwards compatible:

• version stays the default path and is unchanged for existing users.
• version-file points at a .tool-versions file; the action reads the line declared for the packer tool (e.g. packer 1.10.0).
• If both are set, version takes precedence (with a warning), mirroring the behaviour of actions/setup-node's node-version / node-version-file.

action.yml now marks version as required: false (its latest default is retained), so either input may be supplied.

dist/index.js was regenerated with npm run build (tsc + ncc); the large diff is the bundle rebuild — the only functional source change is in src/index.ts.

Validation

• npm run fmt — clean (prettier, no changes)
• npm run build — succeeds (tsc + ncc), regenerated dist/index.js
• Parsing logic unit-checked locally against .tool-versions content (multiple tools, comments, blank lines, missing-tool, multi-version-on-a-line) — all expected results
• Added a version-file leg to the existing actions-self-test.yml self-test workflow, which exercises the rebuilt dist/ end-to-end (uses: ./ → resolve from .tool-versions → assert installed packer version)

Usage

- name: Setup `packer`
  uses: hashicorp/setup-packer@main
  with:
    version-file: .tool-versions

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.8)

Generated-by: Claude Code (Opus 4.8)