phase 44 (part 1): migrate TS callers to /v1/chat + add regression guard

Migrates the four TypeScript /generate callers to the gateway's
/v1/chat surface so every LLM call lands on /v1/usage and Langfuse:

  tests/multi-agent/agent.ts::generate()      provider="ollama"
  tests/agent_test/agent_harness.ts::callAgent provider="ollama"
  bot/propose.ts::generateProposal             provider="ollama_cloud"
  mcp-server/observer.ts (error analysis)      provider="ollama"

Each migration follows the same pattern as the prior generateCloud()
migration (already on /v1/chat from 2026-04-24): replace
`fetch(SIDECAR/generate)` with `fetch(GATEWAY/v1/chat)`, swap the
prompt-style body for OpenAI-compat messages array, extract
content from `choices[0].message.content` instead of `text`.

Same upstream models in every case — gateway is the new home for
the call, transport otherwise unchanged.

Adds scripts/check_phase44_callers.sh — fail-loud regression guard
that exits non-zero if any non-adapter file fetches /generate or
api/generate. Adapter files (crates/gateway, crates/aibridge,
sidecar/) are exempt. Pre-tightening regex flagged prose mentions
in comments; the shipped regex requires `fetch(...)` or
`client.post(...)` shape so comments don't trip it.

Verification:
  bun build mcp-server/observer.ts                       compiles
  bun build tests/multi-agent/agent.ts                   compiles
  bun build tests/agent_test/agent_harness.ts            compiles
  bun build bot/propose.ts                               compiles
  ./scripts/check_phase44_callers.sh                     ✅ clean
  systemctl restart lakehouse-observer                   active

Phase 44 part 2 (deferred):
  - crates/aibridge/src/client.rs:118 still posts to sidecar /generate
    directly. AiClient is the foundational Rust LLM caller used by
    8+ vectord modules; migrating it is a workspace-wide refactor
    that needs its own commit. Plan: keep AiClient as the local-
    transport layer for the gateway's `provider=ollama` arm, but
    introduce a thin `/v1/chat` wrapper for external callers (vectord
    autotune, agent, rag, refresh, supervisor, playbook_memory).
  - tests/real-world/hard_task_escalation.ts: comment mentions
    /api/generate but doesn't actually call it. Comment is left
    intentionally as historical context; regex no longer flags it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/agent_test/agent_harness.ts

@@ -161,16 +161,17 @@ const TOOL_SCHEMA = `Available tools (call by emitting JSON like: {"tool": "name
 // ─── AGENT LOOP ───
 
 async function callAgent(messages: Array<{role: string; content: string}>): Promise<string> {
-  // think:false disables hidden reasoning so all generated tokens go to
-  // visible response. qwen3.5:latest defaults to thinking and silently
-  // burns the token budget otherwise.
-  const r = await fetch(`${SIDECAR}/generate`, {
+  // Phase 44 migration (2026-04-27): /v1/chat instead of direct sidecar
+  // /generate so /v1/usage tracks the call, Langfuse traces it.
+  // think:false still disables hidden reasoning so generated tokens
+  // go to visible response — qwen3.5:latest defaults to thinking.
+  const r = await fetch(`${GATEWAY}/v1/chat`, {
     method: "POST",
     headers: { "content-type": "application/json" },
     body: JSON.stringify({
       model: AGENT_MODEL,
-      prompt: messages.map(m => `${m.role.toUpperCase()}:\n${m.content}`).join("\n\n") + "\n\nASSISTANT:\n",
-      stream: false,
+      provider: "ollama",
+      messages,
       max_tokens: 1500,
       think: false,
     }),
@@ -178,7 +179,7 @@ async function callAgent(messages: Array<{role: string; content: string}>): Prom
   });
   if (!r.ok) throw new Error(`agent ${r.status}: ${(await r.text()).slice(0, 200)}`);
   const j: any = await r.json();
-  return String(j.text ?? j.response ?? "").trim();
+  return String(j?.choices?.[0]?.message?.content ?? "").trim();
 }
 
 function extractToolCall(response: string): { tool: string; args: any } | null {