feat(action): built-in files and editor Actions (#64)

[hammadmajid]

What

• Adds built-in files (ctrl+o, xdg-open/open by runtime.GOOS, detached) and editor (ctrl+e, ${VISUAL:-${EDITOR:-vi}} {path}, attached) Actions.
• action.Opener(goos) names the platform opener, for reuse by other built-ins.
• The editor is resolved by the shell when the Action runs, so it is not fixed at config load.
• Overriding run/key and key = "" work through the existing Merge.
• README documents the built-ins table and overrides.
• A separate commit fixes the three golangci-lint findings from feat: Actions — run a command on the selected Project with a key #62 that kept CI's lint job red.

Footer key hints are still hard-coded (#67 owns them), so the new keys are not shown there yet.

Verification

• gofmt, go vet and golangci-lint are clean; go test passes except the known fish cases of TestScriptExecution.
• Tests cover Opener per OS, the built-in definitions, editor precedence (VISUAL, EDITOR, vi) by running the real command with stand-ins, path quoting, overriding and unbinding.
• The binary builds.

Closes #64