[Story] Telemetry for dynamic tool loading

[edelauna]

Context

We need data to know whether dynamic loading is actually helping: how often the threshold is hit, how large the attached set ends up after the rolling window, how often the model calls `mcp_load`, and which ranker tier fired. Without telemetry we can't tune the defaults.

Depends on #578, #579, #580.

Developer Notes

• Events to emit (via the existing telemetry pipeline — check `src/services/telemetry/`):
  • `dynamic_tools.task_start`: `{ scopedCount, attachedCount, ranker: "embeddings" | "bm25" }`
  • `dynamic_tools.mcp_load_invoked`: `{ taskTurn, queryLength, returnedCount, alreadyAttachedCount }`
  • `dynamic_tools.rolling_window_step`: `{ turn, addedCount, totalAttached }` (only when something changes)
  • `dynamic_tools.ranker_fallback`: `{ from: "embeddings", to: "bm25", reason }`
• Respect existing telemetry opt-out.
• Emit from `src/services/tools/ToolRouter.ts` (most events) and `src/core/tools/mcpLoadTool.ts` (`mcp_load_invoked`).

Acceptance Criteria

• All four event types fire in integration tests
• Opt-out correctly suppresses all events
• No telemetry emitted when dynamic loading is inactive (under threshold)