fix(mcp): auto-detect git repo and branch when omitted from create_session

[Angfr95]

Summary

create_session was silently creating repoless sessions when repo or branch were omitted, even when the MCP server was running inside a Git repository. This fix adds auto-detection so the tool behaves correctly by default.

Changes

• Added detectGitRepo() and detectGitBranch() helpers in create-session.ts that read from git remote get-url origin and git branch --show-current
• Detection only runs when the field is absent — explicit arguments always win
• Non-GitHub remotes and all git failures fall back gracefully to repoless (existing behavior preserved)
• Updated tool and field descriptions to document the new behavior
• Added 8 new tests covering all detection paths and fallback cases

Testing

All 86 tests pass including 17 tests for create-session (9 existing + 8 new).

Fixes #285