Support base branches when creating Agent Host sessions

[meganrogge]

Part of #331567

The Agent Host create_session tool can create independently scoped sessions, but it cannot select the branch that an isolated child session should build on. This prevents an orchestrating session from creating a stack where each session owns one dependent branch and pull request.

GitHub Copilot App's orchestration flow supports this with an optional base branch during session creation.

Proposed behavior

Add an optional baseBranch input to create_session. When provided:

• create the new session in an isolated worktree;
• create its generated working branch from the requested base branch;
• preserve provider-owned configuration when the child uses the same provider;
• do not carry provider-specific configuration across providers.

When omitted, session creation should retain its existing isolation and branch defaults.