fix: cache compiled binary instead of recompiling each run

[hhvrc]

Problem

The action ran go run . on every invocation, recompiling the tool (~15s) each time it's used — in Firmware that's three workflows (release, get-vars, check-changes). setup-go's module cache was supposed to mitigate this but never worked:

##[warning]Restore cache failed: Some specified paths were not resolved, unable to cache dependencies.

The cause: the action's cache-dependency-path is its own go.sum under /home/runner/work/_actions/OpenShock/release-tool/<sha>/go.sum — outside GITHUB_WORKSPACE (.../Firmware/Firmware). GitHub's cache-key path resolution is rooted at the workspace, so it can't resolve a path under _actions/, and the restore fails on every run.

Fix

Build the binary once and cache it, instead of recompiling:

• A content hash of the Go sources (*.go, go.mod, go.sum) is computed in a shell step (works at any absolute path, unlike hashFiles), and used as the cache key. This is correct both for external consumers (pinned by SHA) and for this repo's own uses: ./ self-invocation (where the source changes per commit and github.action_ref would be unreliable).
• On a cache hit, Go setup and compilation are skipped entirely.
• On a miss, setup-go runs with cache: false (no more broken module cache / warning) and go build -o release-tool . produces the binary, which is then cached.
• The run step executes the prebuilt ./release-tool binary; behavior is identical to go run ..
• Added .gitignore for the built release-tool binary so the uses: ./ self-invocation and local builds don't dirty the tree.

Testing

• go build + ./release-tool status produce identical output to go run . status (Next version: 1.6.0 against the Firmware repo).
• action.yml validated as YAML; built binary is git-ignored.

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[stage-review]

Ready to review this PR? Stage has broken it down into 4 individual chapters for you:

	Title
1	Ignore compiled binary in git
2	Implement binary caching logic in action
3	Execute prebuilt binary instead of recompiling
4	Document caching fix in changelog

_{Chapters generated by Stage for commit 5c4aa0f on Jun 25, 2026 10:30am UTC.}

[github-actions]

Change file check

✅ OK — 1 valid change file(s) added in this PR.