feat: add multi-repo support (--repos-dir)

Add the ability to work on multiple git repositories at once by specifying a parent directory instead of a single repo.

New components:

• klausagent/discover.py: discover_repos() walks a parent directory to find all git repositories (with configurable max_depth)
• MultiRepoToolsServer: extends the tool server with list_repos and switch_repo tools so the agent can navigate between repositories
• tools:git-multi CLI subcommand: serves all repos under a directory
• --repos-dir flag for chat and telegram subcommands (mutually exclusive with --repo)

Usage: ./run.sh chat --repos-dir /path/to/projects ./run.sh telegram --repos-dir /path/to/projects

The agent gets two new tools:

• list_repos: shows all available repos and which is active
• switch_repo: switch by name, path, or index

All existing single-repo behaviour is preserved (--repo still works).