* fix(agents): use isPrimaryMode filter for agent picker
Fix#1527: agent picker filtered by mode !== 'subagent' which missed
agents with unexpected mode values. Now uses isPrimaryMode() which
only includes 'primary', 'all', undefined, and null — the semantically
correct set of agents that should appear in the picker.
* fix(agents): use isPrimaryMode consistently across all agent pickers
Updated AgentSelector.tsx to use isPrimaryMode instead of mode !== 'subagent'.
Removed duplicate isPrimaryMode definition from useConfigStore.ts and
imported the shared helper from mobileControlsUtils.
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Co-authored-by: Leonid Skorobogatyy <bash@opencode.itc.local>