fix: restore git-diff mode to fix v1.0.1 Vela regression

[CodeYogiCo]

What broke

PR #5 removed scripts/start.sh and changed the Docker entrypoint from /start.sh to /plugin directly. That shell script was responsible for running git fetch + git diff --unified=0 and piping the result into the plugin. Without it, PARAMETER_DIFF_SOURCE defaults to stdin which is now empty → 0 of 0 changed instructions covered.

Fix

• New internal/plugin/gitdiff/diff.go — runs git fetch --no-tags origin <base> then git diff --unified=0 origin/<base> internally, reproducing what start.sh used to do.
• PARAMETER_DIFF_SOURCE=git — explicit opt-in for any CI.
• Auto-detect for Vela — when PARAMETER_DIFF_SOURCE is unset but VELA_PULL_REQUEST_TARGET is present, the runner automatically switches to git mode. Existing Vela pipelines recover with zero config changes.
• PARAMETER_BASE_BRANCH — explicit override for non-Vela CIs using git mode.

Gap test added

TestDefaultRunner_Run_DiffSourceAutodetect_VelaTarget covers the exact scenario that slipped through: no PARAMETER_DIFF_SOURCE, VELA_PULL_REQUEST_TARGET set, empty stdin. Without the fix this test would have failed when PR #5 landed, catching the regression before merge.

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Generated by Claude Code

[github-actions]

🛡️ Patch Coverage Report

Scope: changed lines only — the code this PR adds or edits, not whole files or the repo. It answers one thing — did your tests run the code you just touched?

Diff coverage: 73% 🟡 — 38 of 52 changed instructions covered

Metric	Value
🟢 Covered instructions	38 (73%)	changed code your tests executed
🔴 Missed instructions	14 (27%)	changed code your tests never ran
📈 Tracked changed lines	52 (40%)	lines the coverage tool could measure
⚪ Untracked changed lines	78 (60%)	comments, blanks, declarations

_{Lines = the source lines you changed. Instructions = the executable units the coverage tool counts inside those lines — one line can hold several (e.g. JaCoCo bytecode), so the two counts differ.}

Coverage by file

File	Diff coverage	Covered / Missed
internal/plugin/gitdiff/diff.go	🟡 50%	13 / 13
internal/plugin/runner.go	🟢 96%	25 / 1

_{1 changed file(s) with no measurable lines (config, docs, generated, or test-only) omitted.}

🔍 Uncovered lines (14)

--- internal/plugin/gitdiff/diff.go:42
	diffArgs := []string{"--no-pager", "diff", "--unified=0", "origin/" + l.baseBranch}
--- internal/plugin/gitdiff/diff.go:43
	if l.module != "" {
--- internal/plugin/gitdiff/diff.go:44
		diffArgs = append(diffArgs, l.module)
--- internal/plugin/gitdiff/diff.go:45
	}
--- internal/plugin/gitdiff/diff.go:46
	logrus.Infof("running git %v", diffArgs)
--- internal/plugin/gitdiff/diff.go:47
	diffCmd := exec.Command("git", diffArgs...) //nolint:gosec // args are controlled internally, not user input
--- internal/plugin/gitdiff/diff.go:48
	var stdout, stderr bytes.Buffer
--- internal/plugin/gitdiff/diff.go:49
	diffCmd.Stdout = &stdout
--- internal/plugin/gitdiff/diff.go:50
	diffCmd.Stderr = &stderr
--- internal/plugin/gitdiff/diff.go:51
	if err := diffCmd.Run(); err != nil {
--- internal/plugin/gitdiff/diff.go:52
		return nil, errors.Wrapf(err, "git diff failed: %s", stderr.String())
--- internal/plugin/gitdiff/diff.go:53
	}
--- internal/plugin/gitdiff/diff.go:55
	return &stdout, nil
--- internal/plugin/runner.go:137
		changedSourceLinesSource = diffReader

_{🤖 Generated by pull-request-code-coverage — coverage for changed lines only.}