feat(prd-1): implement M6 - MCP server platform routing

- Add platform detection to prepareMcpConfig() via detectPlatform()
- Skip GitHub public MCP server (ghcr.io/github/github-mcp-server) on Forgejo
- Skip checkActionsReadPermission on Forgejo (workflow token has full access)
- Reuse existing github-actions-server.ts for both platforms (no duplication)
- Add 6 Forgejo-specific tests to install-mcp-server.test.ts
- Updated PRD checkboxes for M6

Progress: 67% complete (6/9 milestones) - M7 now unblocked

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

src/mcp/install-mcp-server.ts

@@ -4,6 +4,7 @@ import type { GitHubContext } from "../github/context";
 import { isEntityContext } from "../github/context";
 import { Octokit } from "@octokit/rest";
 import type { AutoDetectedMode } from "../modes/detector";
+import { detectPlatform } from "../platform/detector";
 
 type PrepareConfigParams = {
   githubToken: string;
@@ -66,6 +67,10 @@ export async function prepareMcpConfig(
   try {
     const allowedToolsList = allowedTools || [];
 
+    // Detect platform (github vs forgejo)
+    const platform = detectPlatform();
+    const isForgejo = platform === "forgejo";
+
     // Detect if we're in agent mode (explicit prompt provided)
     const isAgentMode = mode === "agent";
 
@@ -167,21 +172,31 @@ export async function prepareMcpConfig(
       hasWorkflowToken;
 
     if (shouldIncludeCIServer) {
-      // Verify the token actually has actions:read permission
-      const actuallyHasPermission = await checkActionsReadPermission(
-        process.env.DEFAULT_WORKFLOW_TOKEN || "",
-        owner,
-        repo,
-      );
+      // On Forgejo, the workflow token has full repo access — skip the
+      // GitHub-specific permission probe which checks for 'actions: read'.
+      // On GitHub, verify the token actually has actions:read permission.
+      let includeCIServer = isForgejo;
 
-      if (!actuallyHasPermission) {
-        core.warning(
-          "The github_ci MCP server requires 'actions: read' permission. " +
-            "Skipping CI server installation. " +
-            "To enable CI status checks, add 'actions: read' to your workflow permissions. " +
-            "See: https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token",
+      if (!includeCIServer) {
+        const actuallyHasPermission = await checkActionsReadPermission(
+          process.env.DEFAULT_WORKFLOW_TOKEN || "",
+          owner,
+          repo,
         );
-      } else {
+
+        if (!actuallyHasPermission) {
+          core.warning(
+            "The github_ci MCP server requires 'actions: read' permission. " +
+              "Skipping CI server installation. " +
+              "To enable CI status checks, add 'actions: read' to your workflow permissions. " +
+              "See: https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token",
+          );
+        } else {
+          includeCIServer = true;
+        }
+      }
+
+      if (includeCIServer) {
         baseMcpConfig.mcpServers.github_ci = {
           command: "bun",
           args: [
@@ -200,7 +215,9 @@ export async function prepareMcpConfig(
       }
     }
 
-    if (hasGitHubMcpTools) {
+    // The GitHub public MCP server (ghcr.io/github/github-mcp-server) is
+    // GitHub-only — it doesn't work with Forgejo instances.
+    if (hasGitHubMcpTools && !isForgejo) {
       baseMcpConfig.mcpServers.github = {
         command: "docker",
         args: [