Coalesce concurrent connectToAgent() calls in SessionManager

## Summary

`SessionManager.connectToAgent(agentName)` should coalesce concurrent calls for the same agent behind a shared pending promise instead of allowing duplicate in-flight connects.

## Why

When multiple startup/view-hydration paths trigger `connectToAgent()` at nearly the same time, the extension can race into duplicate spawn/connect/session creation work. This creates flakiness and slows local iteration/debugging.

## Proposal

- track in-flight `connectToAgent(agentName)` operations in a per-agent map
- if a second call arrives for the same agent while the first is still pending, return the same promise
- clear the pending entry on resolve/reject

## Acceptance criteria

- only one underlying connect/spawn/session-new path runs for concurrent same-agent calls
- parallel callers all receive the same eventual result
- rejection cleanup works correctly
- regression test proves only one connection flow occurs under `Promise.all([connectToAgent("Hermes Agent"), connectToAgent("Hermes Agent")])`

## Notes

This is both a correctness fix and a developer-velocity improvement because duplicate connects make the VS Code session state harder to reason about during local iteration.