harness: pin bridge conversation to local-conv-487 (CONVERSATION_ID constant, --agent omitted for pinned convs)

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Co-Authored-By: Letta Code <noreply@letta.com>

src/letta.zig

@@ -3,6 +3,11 @@ const Io = std.Io;
 
 const LETTA_PATH = "/home/pierre/.bun/bin/letta";
 
+/// Conversation the matrix bridge routes messages into. Pinned to a specific
+/// conversation ID (not "default") so a tainted/default conversation can be
+/// swapped without touching the bridge.
+pub const CONVERSATION_ID = "local-conv-487";
+
 /// Runs `letta -n <agent> -p <prompt>`, captures everything it writes to
 /// stdout, and returns it as an owned slice (caller frees it).
 pub fn infer(
@@ -11,8 +16,10 @@ pub fn infer(
     agent: []const u8,
     prompt: []const u8,
 ) ![]u8 {
+    _ = agent; // unused: pinned conversation implies the agent on the CLI side
     var child = try std.process.spawn(io, .{
-        .argv = &.{ LETTA_PATH, "--agent", agent, "-p", prompt, "--conversation", "default", "--toolset", "default" },
+        // Pinned conversation: --agent must be omitted for non-default conversations.
+        .argv = &.{ LETTA_PATH, "-p", prompt, "--conversation", CONVERSATION_ID, "--toolset", "default" },
         .stdout = .pipe,
         .stderr = .inherit, // letta's error output goes straight to your terminal
     });