feat(cache): split Client/CacheClientBound into capability interfaces

Decompose cache.Client and cache.CacheClientBound into IntentReader,
IntentWriter, RunningStore, and InstanceLifecycle capability
interfaces (recomposed via embedding), regenerate mocks per
capability, and narrow read-only consumers (forEachIntent,
populateSensitivePathIndex) to the bound reader interface.

Make config-server's no-op writes explicit via a standalone
noopIntentWriter composed with *ConfigServerCache in
Server.createCacheClient, instead of burying no-ops inside
ConfigServerCache's own methods.

Also extract LocalCache.decodeIntent to dedup repeated decode logic,
and relocate config-server's ImportConfigAdapter implementation
(documentImporter/mergeConfigBlobs) to pkg/tree/importer/configserver
alongside its sibling tree importers.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: steiler