feat(prd-1): implement M1 - platform detection and action.yml modifications

- Add src/platform/detector.ts with 3-tier detection (explicit input,
  FORGEJO env var, GITHUB_SERVER_URL heuristic)
- Add 'platform' input to action.yml (auto/github/forgejo)
- Add 'Detect Platform' shell step as first composite action step
- Split bun installation: oven-sh/setup-bun for GitHub, curl fallback
  for Forgejo runners
- Condition 'Revoke app token' step to skip on Forgejo
- Pass resolved PLATFORM env var to run step
- Add 14 unit tests covering all detection paths

Progress: M1 complete (1/9 milestones) - M2 and M5 now unblocked

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/platform/detector.ts

@@ -0,0 +1,29 @@
+/**
+ * Platform detection for GitHub vs Forgejo environments.
+ *
+ * Detection priority:
+ * 1. PLATFORM env var (set by action.yml detect step or explicit input)
+ * 2. FORGEJO env var (set by Forgejo runner v7.0+)
+ * 3. GITHUB_SERVER_URL heuristic (non-github.com = Forgejo)
+ * 4. Default to GitHub
+ */
+
+export type Platform = "github" | "forgejo";
+
+export function detectPlatform(): Platform {
+  // 1. Explicit platform from action.yml detect step or user input
+  const platformEnv = process.env.PLATFORM;
+  if (platformEnv === "forgejo") return "forgejo";
+  if (platformEnv === "github") return "github";
+
+  // 2. FORGEJO env var (set by Forgejo runner v7.0+)
+  if (process.env.FORGEJO === "true") return "forgejo";
+
+  // 3. GITHUB_SERVER_URL heuristic — if not github.com, likely Forgejo/Gitea
+  const serverUrl = process.env.GITHUB_SERVER_URL || "";
+  if (serverUrl && !serverUrl.includes("github.com")) {
+    return "forgejo";
+  }
+
+  return "github";
+}