IB-WP-0020-T01: routing config schema and parser

Add a small YAML routing config schema (schema_version 1) and a
parser-only loader at src/infospace_bench/routing_config.py. The
loader validates the declarative shape — task_types with candidates,
optional per-task quality_floor, optional default_quality_floor,
optional ledger_path, optional stage_to_task_type override map — and
refuses bad shapes before any network or workspace work happens.

Supported provider names: openrouter, claude_code, openai, gemini.
Unknown providers, missing required candidate fields, out-of-range
quality floors, negative max_cost_per_1k, duplicate candidate ids
within a task type, and non-mapping stage_to_task_type all raise
focused InfospaceError codes that callers can pattern-match.

docs/routing-config.md documents the schema with two annotated
examples (OpenRouter-only two-tier, and adaptive with a ClaudeCode
baseline) plus the full "what fails fast" list.

16 parser tests cover happy-path round-trip, file load, missing file,
malformed YAML, and every validation surface (wrong/missing schema
version, empty task_types, empty candidates, missing required fields,
unsupported provider, negative cost, out-of-range quality_floor,
duplicate ids, non-mapping stage_map, non-string ledger_path).

T02 will turn a RoutingConfig into a live llm-connect RoutingPolicy /
AdaptiveRoutingPolicy with constructed LLMAdapter instances.

160 tests pass, 1 skipped.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workplans/IB-WP-0020-provider-routing-cli.md

@@ -4,7 +4,7 @@ type: workplan
 title: "Provider Routing CLI Integration"
 domain: markitect
 repo: infospace-bench
-status: todo
+status: active
 owner: markitect
 topic_slug: markitect
 created: "2026-05-18"
@@ -70,7 +70,7 @@ already-closed IB-WP-0018.
 
 ```task
 id: IB-WP-0020-T01
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "39597441-22ab-4dcf-b68d-b045823a9374"
 ```