feat(announcements): remove threads posting and peerlist reminder (#23)

## Summary

- Removes automated announcement posting to Threads (moving to manual cross-posting as part of social engagement strategy)
- Removes Peerlist from the manual posting reminder (moving to social engagement rather than product announcements)
- Updates system prompt and JSON schema descriptions to reflect the current set of platforms

## Test Plan

- [ ] Trigger an announcement and confirm Threads is no longer called
- [ ] Confirm the alert in the response no longer mentions Peerlist
- [ ] Confirm lint and build pass (verified locally)

✨ This issue was created with help from Hikari~ 🌸

Reviewed-on: #23
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

server/src/config/announcements.ts

@@ -24,7 +24,7 @@ Platform-specific requirements:
 - Include clear calls to action
 - The same content will be used for Discord, Reddit, Ko-fi, and Patreon, so make it work well for all these platforms
 
-**Threaded (for Threads, Twitter, Bluesky, and Mastodon):**
+**Threaded (for Twitter, Bluesky, and Mastodon):**
 - Break content into a thread of individual posts
 - Each post should be under 280 characters (to work for Twitter's limit, which is the most restrictive)
 - Posts should flow naturally from one to the next
@@ -32,14 +32,14 @@ Platform-specific requirements:
 - Make the first post compelling to encourage thread reading
 - Do NOT include post numbers or thread indicators (e.g., "1/5" or "🧵")
 - Plain text format (no markdown)
-- The same thread will be used for Threads, Twitter, Bluesky, and Mastodon
+- The same thread will be used for Twitter, Bluesky, and Mastodon
 
-**Plaintext (for LinkedIn, Facebook, and Peerlist):**
+**Plaintext (for LinkedIn and Facebook):**
 - Plain text format (no markdown)
 - Professional yet friendly tone, conversational style suitable for a broader audience
 - Include 3-5 relevant hashtags
 - Keep it concise but informative
-- The same content will be used for LinkedIn, Facebook, and Peerlist
+- The same content will be used for LinkedIn and Facebook
 
 **Universal requirements:**
 - All announcements must include a call to action to donate (https://donate.nhcarrigan.com)
@@ -71,13 +71,13 @@ const announcementJsonSchema = {
       type:     "object",
     },
     plaintext: {
-      description: "Plain text announcement for LinkedIn, Facebook, and Peerlist (shared content). Should be professional yet friendly, conversational style suitable for a broader audience. Include 3-5 relevant hashtags and calls to action for donating and joining Discord.",
+      description: "Plain text announcement for LinkedIn and Facebook (shared content). Should be professional yet friendly, conversational style suitable for a broader audience. Include 3-5 relevant hashtags and calls to action for donating and joining Discord.",
       maxLength:   1900,
       minLength:   100,
       type:        "string",
     },
     threaded: {
-      description: "Array of individual posts that form a thread. Will be used for Threads, Twitter, Bluesky, and Mastodon. Each post should be under 280 characters (Twitter's limit) and flow naturally from one to the next.",
+      description: "Array of individual posts that form a thread. Will be used for Twitter, Bluesky, and Mastodon. Each post should be under 280 characters (Twitter's limit) and flow naturally from one to the next.",
       items:       {
         description: "A single post in the thread (max 280 characters, no post numbers or thread indicators)",
         maxLength:   280,