Add MAX_THREAD_POSTS config for thread context limit

.env.example

···
 # RESPONSIBLE_PARTY_BSKY="DID:... or example.bsky.app, no @symbol"
 # EXTERNAL_SERVICES="Letta, Railway, Google Gemini 2.5-pro"
 # PRESERVE_MEMORY_BLOCKS=true
+# MAX_THREAD_POSTS=25

README.md

···
 - **`RESPONSIBLE_PARTY_BSKY`**: the DID or bluesky handle of the responsible party
 - **`EXTERNAL_SERVICES`**: a comma-separated list of external tools and services your agent relies on outside of Bluesky (e.g., "Letta, Railway, Google Gemini 2.5-pro"). This information is added to your agent's autonomy declaration record on the PDS and included in the agent's memory for transparency.
 - **`PRESERVE_MEMORY_BLOCKS`**: a boolean for controlling if your agent's memory blocks can be overridden if you run `deno task mount` more than once. Setting this value to **`true`** will allow your agent's version of those memory blocks to persist if they already exist. This is false by default.
+- **`MAX_THREAD_POSTS`**: maximum number of posts to include when fetching thread context (5-250). When a thread exceeds this limit, it will include the root post, a truncation indicator, and the most recent N posts. Default: 25

utils/agentContext.ts

···
   const value = Number(Deno.env.get("MAX_THREAD_POSTS"));
 
   if (isNaN(value) || value < 5 || value > 250) {
-    return 2;
+    return 25;
   }
 
   return Math.round(value);