feat(social): implement Reddit tools — search, posts, submit

[EngineerProjects]

Context

Stub is at `internal/tools/social/reddit/tool.go`. All methods return `ErrNotImplemented`. `IsEnabled() = false` so tools are invisible to agents until implemented.

Tools to implement

Tool	Description
`reddit_search`	Full-text search across Reddit or within a subreddit
`reddit_posts`	Fetch hot/new/top/rising from any subreddit
`reddit_post`	Fetch a single post + comment thread
`reddit_submit`	Submit a link or text post
`reddit_comment`	Post a comment on a submission
`reddit_subreddit`	Fetch subreddit info (rules, description, subscribers)

Auth

Reddit OAuth2 — script app type (username+password flow) for personal use, web app for multi-user.

Required env vars:

• `REDDIT_CLIENT_ID`
• `REDDIT_CLIENT_SECRET`
• `REDDIT_USERNAME` (script apps)
• `REDDIT_PASSWORD` (script apps)

API

Official Reddit REST API v1: `https://www.reddit.com/dev/api/\`

Token endpoint: `POST https://www.reddit.com/api/v1/access_token\`
Auth: Basic(client_id, client_secret) + form body `grant_type=password&username=&password=`

Rate limit: 100 requests/minute per OAuth client.

Implementation notes

• Token refresh: access tokens expire in 1h → store expiry, refresh before each request
• User-Agent header is mandatory: `nexus-engine/1.0 by `
• `reddit_submit` and `reddit_comment` require `RequiresPermission: true`
• Keep `IsEnabled() = false` on all tools until env vars are present (check in `IsEnabled()`)