feat(desktop): worktree agent data sync, retired persona cleanup, mcp_command reconciliation

[wpfleger96]

Worktree dev builds launched with SPROUT_SHARE_IDENTITY=1 show empty agent panels because each worktree gets its own ~/Library/Application Support/<identifier>/ data directory that starts with empty defaults. Meanwhile, users who ran Sprout before the Solo/Kit/Scout transition still carry 6 retired personas (Builder, Orchestrator, Planner, Refactor, Researcher, Reviewer) that clutter their persona list. And agents created before #584 still have mcp_command = "sprout-mcp-server" stored, causing goose to register the full 49-tool messaging MCP instead of using the CLI.

This PR adds three on-launch data management features plus a frontend fix:

Worktree agent-data sync via symlinks

sync_shared_agent_data() in migration.rs creates symlinks for managed-agents.json, personas.json, and teams.json from the current worktree's data directory to the canonical xyz.block.sprout.app.dev data directory. All dev instances share the same physical files — edits in any worktree (persona deletions, agent config changes, team updates) are immediately visible to all others, matching the mental model that "dev Sprout is one logical instance."

Guards: SPROUT_SHARE_IDENTITY=1, valid SPROUT_PRIVATE_KEY, canonical dir differs from current dir (via fs::canonicalize for case-insensitive APFS), canonical dir exists. Idempotent: correct symlinks are preserved, wrong/broken symlinks are replaced, regular files from prior launches are replaced with symlinks.

agents/packs/ is intentionally excluded — recursive directory symlink is out of scope. Pack personas appear in the worktree but ACP processes that read pack files at runtime may fail; install packs in the worktree separately if needed.

Atomic, symlink-preserving writes

All three agent data save functions (save_managed_agents, save_personas, save_teams) now use a shared atomic_write_json helper that resolves symlinks via canonicalize before doing the tmp-file + rename. Previously, managed-agents.json used an atomic write pattern that would destroy symlinks (rename replaces the symlink entry), while personas.json and teams.json used non-atomic fs::write. Now all three are consistent: atomic writes that preserve symlinks.

Retired persona migration

migrate_retired_personas() in personas.rs appends " (retired)" to retired personas' display names, sets is_active = false, and refreshes updated_at. Called from merge_personas() after the existing demotion pass.

Always soft-deprecates, never deletes. Removing records would orphan persona_id references in managed-agents.json and teams.json — resolve_persona_env fails closed on missing personas, blocking agent spawn. Preserves the user's display name (a persona renamed to "My Reviewer" becomes "My Reviewer (retired)"). Idempotent: already-retired records (suffix present AND is_active = false) are skipped on subsequent launches.

The frontend's isPersonaActive previously returned true for all non-builtins regardless of isActive, so retired personas still appeared in the agents panel. Fixed by checking persona.isActive directly and passing filtered libraryPersonas to UnifiedAgentsSection instead of raw query data.

mcp_command provider reconciliation

reconcile_provider_mcp_commands() in migration.rs reads managed-agents.json on every launch, looks up each agent_command in the discovery table via known_acp_provider(), and patches mcp_command to the provider's canonical value. Goose/claude/codex get "" (no MCP server), sprout-agent gets "sprout-dev-mcp", unknown/custom agents are left untouched. This closes the gap left by #584 which only fixed creation-time defaults — existing agents with stale mcp_command = "sprout-mcp-server" now get corrected automatically.

Other changes

• Extracted patch_json_records() helper used by reconcile_mcp_commands_in_file — eliminates duplicated read→parse→mutate→write boilerplate, includes parse-failure logging
• Simplified RETIRED_PERSONAS constant from (&str, &str, &str) to (&str, &str) by dropping the unused display name field
• Removed dead legacy migration code (migrate_legacy_data_dir, migrate_file, LEGACY_FILES) after upstream deletion in ab71ade9
• Wired data management calls into lib.rs setup hook: sync_shared_agent_data → reconcile_provider_mcp_commands → ... → restore_managed_agents_on_launch