refactor(runtime): one compact-runtime selection rule for route and dispatch

The chat route repeated the compact (clean v3) eligibility decision inline
to prepare the turn's context resolution, while the agent loop dispatched
on the contract stamp set by a separate, later condition. The two could
drift, and already disagreed for a user whose privileges demote the turn
to plain chat: the route prepared a compact resolution that no compact
runtime used.

src/agent_runtime/runtime_selection.py (no imports) now owns the rule:

- uses_compact_preview_runtime(): clean route requested, contract policy
  enabled, agent mode, agent permitted, not an image generation session.
- is_compact_preview_contract() and COMPACT_PREVIEW_MODE for the stamp.

The route evaluates the rule once, before context preparation, where all
of its facts are final (the agent privilege is read through the same
_request_privileges helper the later enforcement uses). That one value
gates the typed context resolution and is the _clean_v3_preview flag that
stamps the contract; inside the agent-contract branch it equals the
previous condition, so stamping behavior is unchanged. The agent loop
dispatches through is_compact_preview_contract(), and the compact runtime's
MODE is the shared constant.

A route-level matrix drives the real agent loop and asserts that route
preparation and compact dispatch agree for compact, escalated, configured
compact/full, regular, TUI, privilege-denied and image-generation turns.

src/clean_agent_preview.py

@@ -21,6 +21,7 @@ import httpx
 import jsonschema
 
 from src.context_compactor import prune_multimodal_images, trim_for_context
+from src.agent_runtime.runtime_selection import COMPACT_PREVIEW_MODE
 from src import agent_runs
 from src.agent_evidence import command_has_mutation_effect, workspace_artifact_is_usable
 from src.tool_capabilities import ToolEffect, ToolRunSecurityContext, capabilities_for_action
@@ -47,7 +48,7 @@ from src.model_profiles import (
 )
 
 ENDPOINT_ID = 'cleanv3'
-MODE = 'clean_compact_v3_preview'
+MODE = COMPACT_PREVIEW_MODE
 
 
 class ProviderStreamError(Exception):