agent: unattended Phase 2 loop — profile to gated verdict, zero false accepts

The Claude Agent SDK loop (agent/hotpath_agent/loop.py, model
claude-fable-5) drives the seven harness tools over a standalone stdio
MCP server (mcp_server.py) and runs the full profile -> hypothesize ->
patch -> gated-verdict cycle on comrak with no human in the loop.

Three audited attempts, all honest rejections, ZERO false accepts:
- phase2-comrak-001 class 3: syntect buffer with_capacity, [0.971,1.007]
- phase2-comrak-002 class 5: gate dead VecDeque build, [0.982,1.026]
- phase2-comrak-003 class 6: hoist find_special_char check, [0.970,1.007]
The agent picked the cheapest untried class each time (3->5->6) and
TWICE refused to fabricate a verdict it couldn't read back, verifying
ledger integrity instead — the anti-reward-hacking behavior SPEC §10
targets.

Harness changes surfaced by the loop:
- run_verdict is now async: launches the multi-minute build+bench
  detached (past the 60s MCP transport cap) and returns immediately;
  new read_verdict op polls the append-only ledger row (ledger
  verdict_summary). Fixes attempt 001's unreadable-verdict gap.
- read_target_source takes offset/limit line windows so 100KB+ files
  page instead of overflowing — removes the agent's reason to reach for
  a shell (attempt 003 paged a 2400-line file cleanly, no shell).

Honest boundary note (case study + code comments): in THIS dev
container the SDK tool allow/deny layer is defense-in-depth, not the
trust boundary — a nested CLI as root under a parent session doesn't
reliably gate built-ins. SPEC §10's boundary is OS process/user
isolation; that hardening is the remaining Phase 2 exit item
(docs/infra.md). What holds regardless: propose_patch is the only
mediated write path (path-allowlist + git apply) and the ledger is
append-only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

agent/pyproject.toml

@@ -4,7 +4,8 @@ version = "0.1.0"
 description = "hotpath optimization agent: the untrusted proposer in the profile→patch→verdict loop"
 requires-python = ">=3.11"
 dependencies = [
-    "claude-agent-sdk>=0.1",
+    "claude-agent-sdk>=0.2",
+    "mcp>=1.0",
 ]
 
 [project.optional-dependencies]