docs(plans): address PR #73 review — fix deprecated set-output + MD040

CodeRabbit fact-checked both findings against current code; both real.

- T1 (Major): replaced deprecated `::set-output` (deprecated 2022-10 per
  GitHub changelog) with the `GITHUB_OUTPUT` environment file approach
  in the package-manager-detector wrapper sketch (Q2 resolution). Used
  `appendFileSync(process.env.GITHUB_OUTPUT, ...)` per current GitHub
  Actions docs.
- T2 (Minor, MD040): added `text` language tag to the Q3 resolution
  pseudo-code block so markdownlint-cli2 stops flagging the unlanguaged
  fence.

Author: SutuSebastian

docs/plans/github-marketplace-action.md

@@ -81,11 +81,13 @@ These are the design questions the plan-PR resolves before impl starts. Each get
   ```javascript
   import { detect } from "package-manager-detector/detect";
   import { resolveCommand } from "package-manager-detector/commands";
+  import { appendFileSync } from "node:fs";
 
   const pm = await detect();
   const agent = pm?.agent ?? "npm";
   // resolveCommand handles `npm exec` vs `pnpm exec` vs `bun x` vs `yarn dlx` — Q3 lookup-table for free
-  console.log(`::set-output name=agent::${agent}`);
+  // Write to GITHUB_OUTPUT env file (`::set-output` deprecated since 2022-10).
+  appendFileSync(process.env.GITHUB_OUTPUT, `agent=${agent}\n`);
   ```
 
   Precedence: `package-manager:` input still wins when set (overrides detection). When unset, the library's default strategy order applies. Multiple-lockfile case is handled by the library; we surface its result + log a warning if ambiguous.
@@ -98,7 +100,7 @@ These are the design questions the plan-PR resolves before impl starts. Each get
 
   **Resolution: project-installed first, download-and-execute fallback.** Maps directly onto `package-manager-detector`'s two `resolveCommand` intents — `'execute-local'` (already installed) vs `'execute'` (download and run). No hand-rolled per-agent branching; the library resolves `npx` / `bunx` / `pnpm dlx` / `yarn dlx` itself.
 
-  ```
+  ```text
   if version-input is set:
     intent = 'execute'                  # forced download with pinned version
     cli = resolveCommand(agent, 'execute',       ['codemap@<version>'])