fix: prevent agent settings from being silently deleted on channel deletion

[wesbillman]

Summary

• Disable auto-deletion of managed agents on channel deletion. cleanupChannelAgents and cleanupManagedAgentIfOrphaned relied on relay kind:10100 events to determine whether an agent was "orphaned" — but relay data can be stale/incomplete, causing agents that are still active in other channels to be incorrectly deleted. This wiped all customized settings (ENV vars, MCP config, parallelism, system prompts). Both functions and their call sites have been removed entirely.

• Make save_managed_agents use atomic writes. Previously used bare fs::write which can leave a corrupted/partial file if the process crashes mid-write. Now writes to a .json.tmp file first, then atomically renames it into place.

• Remove legacy migration code. The com.wesb.sprout → xyz.block.sprout.app data directory migration is no longer needed — all existing users have been migrated. Removes migration.rs and its call site entirely.

Root cause

When a channel is deleted, the frontend called cleanupChannelAgents(channelId), which:

1. Fetched all channel members
2. Queried relay kind:10100 events to check if each agent exists in other channels
3. Deleted managed agent records for any agent it considered "orphaned"

The relay data used in step 2 can be stale or incomplete — an agent that IS still active in another channel may appear orphaned because its kind:10100 event hasn't been indexed yet. This causes the agent record (with all its customized settings) to be deleted.

Changes

File	Change
desktop/src/features/channels/cleanupChannelAgents.ts	Deleted — no-op cleanup functions removed entirely
desktop/src/features/channels/hooks.ts	Removed cleanup imports, try/catch blocks, and removeChannelMemberWithManagedAgentCleanup helper
desktop/src/features/channels/ui/useMembersSidebarActions.ts	Replaced cleanup helper with direct removeChannelMember call
desktop/src-tauri/src/managed_agents/storage.rs	Atomic write-to-temp + rename for save_managed_agents
desktop/src-tauri/src/migration.rs	Deleted — legacy directory migration no longer needed
desktop/src-tauri/src/lib.rs	Removed mod migration and migrate_legacy_data_dir call

Net: -397 lines

Test plan

• All 11 pre-commit hooks pass (rust-fmt, web-check, desktop-tauri-fmt, desktop-check, web-build, mobile-check, desktop-build, desktop-tauri-check, mobile-test, rust-clippy, rust-tests)
• Manual verification: edit agent settings → delete a channel containing that agent → confirm settings are preserved
• Manual verification: create agent in multiple channels → delete one channel → confirm agent and settings survive

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com