docs(contributing): document reqstool/openspec CLI prerequisites

Neither CLI was documented as a prerequisite anywhere. Also notes the
Claude Code marketplace/plugin prompt contributors will see on first
open, now that .claude/settings.json enables it by default.

Signed-off-by: Jimisola Laursen <jimisola@jimisola.com>

Author: jimisola

CONTRIBUTING.md

@@ -8,6 +8,8 @@ For DCO sign-off, commit conventions, and code review process, see the organizat
 
 - Java 21+
 - Gradle 9.3+ (use the wrapper)
+- [reqstool](https://github.com/reqstool/reqstool-client) (`pipx install reqstool`)
+- [OpenSpec](https://github.com/Fission-AI/OpenSpec) (`npm install -g @fission-ai/openspec`)
 
 ## Setup
 
@@ -16,9 +18,13 @@ git clone https://github.com/reqstool/reqstool-java-gradle-plugin.git
 cd reqstool-java-gradle-plugin
 ```
 
-If using Claude Code, regenerate the `opsx` slash commands and OpenSpec skills
-(`.claude/commands/opsx/`, `.claude/skills/openspec-*`) after cloning — they're
-CLI-generated tool scaffolding, not committed to the repo:
+If using Claude Code, opening this repo will prompt you to confirm adding the `reqstool-ai`
+marketplace and enabling the `reqstool`/`reqstool-openspec` plugins (configured in
+`.claude/settings.json`) — accept the prompt.
+
+Then regenerate the `opsx` slash commands and OpenSpec skills
+(`.claude/commands/opsx/`, `.claude/skills/openspec-*`) — they're CLI-generated tool scaffolding,
+not committed to the repo:
 
 ```bash
 openspec update   # or: openspec init --tools claude --force