Fix branch selector for single message and --no-session mode

- Allow branch selector to open with single user message (changed <= 1 to === 0 check)
- Support in-memory branching for --no-session mode (no files created)
- Add isEnabled() getter to SessionManager
- Update sessionFile getter to return null when sessions disabled
- Update SessionSwitchEvent types to allow null session files
- Add branching tests for single message and --no-session scenarios

fixes #163

packages/coding-agent/CHANGELOG.md

@@ -6,6 +6,12 @@
 
 - **HTML export line numbers**: Read tool calls in HTML exports now display line number ranges (e.g., `file.txt:10-20`) when offset/limit parameters are used, matching the TUI display format. Line numbers appear in yellow color for better visibility. ([#166](https://github.com/badlogic/pi-mono/issues/166))
 
+### Fixed
+
+- **Branch selector now works with single message**: Previously the branch selector would not open when there was only one user message. Now it correctly allows branching from any message, including the first one. This is needed for checkpoint hooks to restore state from before the first message. ([#163](https://github.com/badlogic/pi-mono/issues/163))
+
+- **In-memory branching for `--no-session` mode**: Branching now works correctly in `--no-session` mode without creating any session files. The conversation is truncated in memory.
+
 ## [0.18.1] - 2025-12-10
 
 ### Added