fix(desktop): defer host reads for pending sessions

[liuxiaocs7]

Summary

Keep a newly-created pending Session available to the local chat UI while withholding its id from Runtime Host-backed consumers until the authoritative catalog admits it.

This prevents the initial Session activation race from triggering not_found reads in Session observation, Plan state, context diagnostics, shell runs, terminal, side-chat, and browser workbar integrations. Cached Host Sessions remain readable, and shared Sessions retain Host-backed access without local-owner capabilities.

Fixes #5081

Verification

• npm --workspace @maka/desktop run build:workspace-deps
• npm --workspace @maka/desktop run typecheck
• npm --workspace @maka/desktop run check:architecture (101 tests)
• npx biome check on the changed TypeScript files
• npm run format:check
• npm --workspace @maka/desktop run build:main
• npm --workspace @maka/desktop run test:dist (2536 tests)
• git diff --check

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex diagnosed the pending Session/Runtime Host admission race, implemented the boundary projection and regression tests, and ran verification.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No