feat: add {{BASE_BRANCH_DETECT}} resolver to gen-skill-docs

DRY placeholder for dynamic base branch detection across PR-targeting
skills. Detects via gh pr view (existing PR base) → gh repo view
(repo default) → fallback to main.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/gen-skill-docs.ts

@@ -126,11 +126,33 @@ If \`NEEDS_SETUP\`:
 3. If \`bun\` is not installed: \`curl -fsSL https://bun.sh/install | bash\``;
 }
 
+function generateBaseBranchDetect(): string {
+  return `## Step 0: Detect base branch
+
+Determine which branch this PR targets. Use the result as "the base branch" in all subsequent steps.
+
+1. Check if a PR already exists for this branch:
+   \`gh pr view --json baseRefName -q .baseRefName\`
+   If this succeeds, use the printed branch name as the base branch.
+
+2. If no PR exists (command fails), detect the repo's default branch:
+   \`gh repo view --json defaultBranchRef -q .defaultBranchRef.name\`
+
+3. If both commands fail, fall back to \`main\`.
+
+Print the detected base branch name. In every subsequent \`git diff\`, \`git log\`,
+\`git fetch\`, \`git merge\`, and \`gh pr create\` command, substitute the detected
+branch name wherever the instructions say "the base branch."
+
+---`;
+}
+
 const RESOLVERS: Record<string, () => string> = {
   COMMAND_REFERENCE: generateCommandReference,
   SNAPSHOT_FLAGS: generateSnapshotFlags,
   UPDATE_CHECK: generateUpdateCheck,
   BROWSE_SETUP: generateBrowseSetup,
+  BASE_BRANCH_DETECT: generateBaseBranchDetect,
 };
 
 // ─── Template Processing ────────────────────────────────────