mobile: sync saved computers across devices; faster reconnect

[0xSero]

Purpose

Make the "machines" experience work like T3 Connect: a computer paired on one device shows up on the user's other Apple devices, and reconnecting after launch or foreground is faster.

Key changes

• Computer sync (iOS). Saved Kittylitter and direct-URL computers sync through the existing iCloud KVS bridge. The merge policy lives in Rust (cloud_sync::computers): a last-write-wins element set, so concurrent adds on two devices both survive and removals propagate instead of being resurrected. 8 unit tests.
• What does not sync: Local Studio (grant bound to one device's iroh key), SSH (keys never leave the device), ChatGPT-connected computers (need that device's own sign-in). Covered by SavedServerStoreTests.
• Credentials. Tokens for synced Kittylitter computers move to iCloud Keychain (end-to-end encrypted); Kittylitter hosts authenticate by token alone (alleycat host.rs validate_token). All other tokens stay ThisDeviceOnly, which is now the store default. Token reads are cached in memory instead of reading and rewriting the Keychain on every reconnect.
• Reconnect (shared Rust, both platforms). Concurrent reconnect triggers wait for the in-flight pass instead of returning an empty result (which cleared the "connecting" state early). Foreground account probes run detached with a 10 s per-probe deadline, so a hung remote no longer holds up the reconnect result.
• Remove the add-server first-run coachmark on iOS and Android.
• Build hygiene: cap the shared Cargo target (60 GB default, LITTER_RUST_TARGET_MAX_GB) before Rust builds; host dev profile uses line-table debug info. The target had grown to 243 GB.
• Stop tracking an accidentally committed self-referencing apps/ios/Resources/fs symlink that broke Xcode builds on fresh checkouts.

Verification

• cargo test -p codex-mobile-client --lib cloud_sync → 16 passed; --lib reconnect → 53 passed.
• make ios-sim-fast → BUILD SUCCEEDED; xcodebuild test -only-testing:LitterTests/SavedServerStoreTests → passed.
• App launched on iPad simulator; first-run hint removed.
• Not verified: real two-device iCloud sync (simulator has no iCloud account). Android not built locally (no toolchain on this Mac); CI covers it.

Platform notes

Computer sync is iOS-only: Android has no iCloud. Follow-up for Android would need Google Block Store or Drive app data. Reconnect fixes and the coachmark removal apply to both.

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