fix(client): disable SSE read timeout — caught by personal Worldtree smoke

First manual smoke against personal Worldtree (10.250.50.152:8081)
produced httpx.ReadTimeout mid-stream after the worker_phase
BuildingPrompt event. Root cause: httpx's default 5s read timeout
killed the connection during mimir's thinking phase (LLM streaming
has multi-second idle gaps between SSE events).

Fix at the caller layer (where the AsyncClient is owned):
- cli._amain and tui.on_mount now construct AsyncClient with
  timeout=httpx.Timeout(connect=10.0, read=None, write=10.0, pool=10.0).
  read=None disables the SSE-killing timeout; connect/write/pool
  keep modest timeouts so true network failures still surface
  promptly.

Defense in depth in sse_client.stream_turn:
- ERROR_ROUTING now also catches httpx.ReadTimeout (was just
  ReadError | RemoteProtocolError) and surfaces it as
  SseConnectionDropped, so if a caller misconfigures their client
  the failure is at least a named exception the presenters handle.

Contract amendments (in-place):
- Issue #1: new [compatibility] constraint documents the read=None
  recommendation; ERROR_ROUTING for stream_turn lists ReadTimeout
  alongside ReadError/RemoteProtocolError.
- Issues #3 + #4: AsyncClient construction step now spells out the
  timeout shape explicitly.

Smoke after fix: SSE stream consumed cleanly, agent responded,
[done] turn_id=88 model=qwen3.6-35-a3b duration_ms=2351. Stdout-only
(2>/dev/null) returned clean agent text + exit 0 — INV-002
stdout/stderr split holds end-to-end against real wire. Wire-compat
envelope (personal v0.16.2 vs ratatoskr's v0.19.0 pin) confirmed.

164/164 tests GREEN; ruff clean; all three drift checks clean.

Note: TUI mode not smoke-tested from this CC session (needs a TTY;
operator-side check via `source env.sh && uv run ratatoskr --new
--agent mimir`).

docs/contracts/issues/4.contract.md

@@ -212,7 +212,7 @@ ERROR_ROUTING:
     state_recovery: none
 STEPS:
   1. [setup, flexibility=prescriptive] Validate PRE-001
-  2. [sequential, flexibility=prescriptive] Open AsyncClient: self.client = httpx.AsyncClient(base_url=args.server_url, headers={"Authorization": f"Bearer {args.api_key}"})
+  2. [sequential, flexibility=prescriptive] Open AsyncClient: self.client = httpx.AsyncClient(base_url=args.server_url, headers={"Authorization": f"Bearer {args.api_key}"}, timeout=httpx.Timeout(connect=10.0, read=None, write=10.0, pool=10.0)) — read=None disables the SSE-killing 5s default (issue #1 [compatibility] constraint)
   3. [branch, flexibility=prescriptive] IF args.new:
        TRY: info = await create_session(self.client, args.agent_id)
        ON AgentNotFound | SessionApiFailed | httpx.ConnectError | httpx.ReadTimeout | httpx.TransportError: handle per ERROR_ROUTING (append + exit)