feat: move-diff-logic-within-the-plugin

[CodeYogiCo]

Drop the Vela start.sh entrypoint and run the plugin binary directly (ENTRYPOINT ["/plugin"]). The diff is now produced in-plugin (PARAMETER_DIFF_SOURCE=github) or piped on stdin, so the Vela git-diff wrapper is no longer needed.

• Remove scripts/start.sh; Dockerfile now uses ENTRYPOINT ["/plugin"].
• Drop the redundant --entrypoint /plugin override in the GitHub Actions workflow.
• Run the coverage job on fork PRs (removed the same-repo guard); tolerate the read-only-token comment 403 on forks via continue-on-error.

🤖 Generated with 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: 94% 🟢 — 78 of 83 changed instructions covered

Metric	Value
🟢 Covered instructions	78 (94%)	changed code your tests executed
🔴 Missed instructions	5 (6%)	changed code your tests never ran
📈 Tracked changed lines	83 (17%)	lines the coverage tool could measure
⚪ Untracked changed lines	392 (83%)	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/runner.go	🟢 90%	18 / 2
internal/test/mocks/property_getter.go	🟢 94%	16 / 1
internal/plugin/githubdiff/diff.go	🟢 94%	33 / 2
internal/plugin/sourcelines/unifieddiff/changed_source_loader.go	🟢 100%	11 / 0

_{7 changed file(s) with no measurable lines (config, docs, generated, or test-only) omitted.}

🔍 Uncovered lines (5)

--- internal/plugin/githubdiff/diff.go:70
		return nil, errors.Wrap(readErr, "Failed reading diff response from github")
--- internal/plugin/githubdiff/diff.go:71
	}
--- internal/plugin/runner.go:120
			return errors.Wrap(fetchErr, "Failed fetching diff from github")
--- internal/plugin/runner.go:121
		}
--- internal/test/mocks/property_getter.go:31
			continue

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