Plan agent: 3-tier graceful degradation for Explore and search subagent (#311988)

* Plan agent: graceful degradation when Explore sub-agent is disabled

* feat: 3-tier discovery prompt and fix SwitchAgentTool config mismatch

- buildAgentBody takes (exploreEnabled, searchSubagentEnabled) for
  Explore → search subagent → generic fallback discovery tiers
- SwitchAgentTool uses getExperimentBasedConfig instead of direct
  vscode.workspace.getConfiguration, matching PlanAgentProvider
- Listen for SearchSubagentToolEnabled config changes
- 7 new tests for discovery tiers, config listener, and explore toggle