fix(agent): restore custom Agent selection and session launches

[devRickyyy]

Summary

• restore exact Harness target icon inheritance so custom Workspace Agents remain visible and selectable
• wire WorkspaceAgent launch resolution and activity-projection validation into the production daemon composition root
• bridge the released Kimi Code built-in-to-Extension version-skew window by canonicalizing local:kimi-code to the installed extension:kimi-code target
• preserve the strict agent_extension launch requirement and trusted Extension installation reference
• document the target identity, WorkspaceAgent wiring, and troubleshooting contracts

Root causes

1. Custom Agent directory projection depended on an optional icon resolver that was not wired into DesktopAgentsService; entries without a decorative icon were then dropped before the composer menu and provider rail were projected.
2. Production constructed the WorkspaceAgent aggregate but did not attach its resolver to Agent session creation or its registry store to activity projection. Session creation therefore failed with workspace agent resolver is unavailable, while restored sessions could lose their workspace-agent:* target identity.
3. During Kimi Code externalization, a released/stale renderer could still submit local:kimi-code with provider kimi-code to a daemon that correctly requires the installed Extension identity extension:kimi-code / acp:kimi-code.

Compatibility boundary

The daemon alias is intentionally narrow. It resolves only local:kimi-code, only if the canonical installed Extension target exists, then rewrites target/provider identity before persistence and launch. Missing Extensions and unrelated provider mismatches still fail. The dynamic runtime guard requiring an agent_extension target is unchanged.

Validation

• focused Desktop agent-directory tests: 8/8
• full affected Go packages: service/agent, data/workspace
• production wiring regression tests
• Kimi alias positive, missing-installation, and provider-mismatch regressions
• pnpm lint:go
• pnpm typecheck (27 packages)
• pnpm check:agent-host-boundary
• Desktop production build and renderer CSS contracts
• full Go workspace tests: 14 lanes
• Go workspace build
• pnpm check:changed: 18/18 lanes
• pre-push pnpm check:changed -- --push-ready: 20/20 lanes (passed twice)