mobile: preserve runtime-only thread selection

[0xSero]

Purpose

Replace stale PR #177 with a current-main, parity-safe fix for runtime-only thread launches. Selecting OpenCode or another non-Codex runtime must not fall back to Codex merely because no explicit model override was chosen.

Key changes

• centralize the iOS runtime/model launch projection and use it in all three launch paths
• preserve the same runtime-only state and request projection on Android
• project the resolved runtime into the initial Rust thread snapshot before the first session-summary projection
• retain the runtime routing map needed for later resume/turn operations
• omit fix(ios): show new opencode sessions immediately #177’s navigation hunk because that behavior already landed on main

Verification

• focused Android selection tests pass
• full Android debug unit suite passes
• focused iOS tests pass on the iPhone 17 Pro simulator: 3 tests, 0 failures
• rustfmt --check and git diff --check pass

Remaining acceptance

The pristine Codex submodule cannot compile the focused Rust test until the repository patch stack is applied (JsonRpcWire drift). CI must exercise the normal sync/patch lane. A physical-device OpenCode creation remains the final end-user acceptance check.

Supersedes #177.

[0xSero]

Cleanup review status: the current-main replacement is code-complete and focused iOS/Android tests pass, but it remains draft until the user-facing acceptance surface is proven. Required final check: create a runtime-only OpenCode thread on a live host from both mobile launch paths and confirm the new thread immediately opens and remains projected as OpenCode after refresh/reconnect. The focused Rust test is also awaiting the normal synced Codex patch stack in CI because a bare local submodule hit the pre-existing JsonRpcWire incompatibility.