fix: clear ghost status-bar lines on terminal resize (#4960)

* feat: add /branch (/fork) command for session branching

Inspired by Claude Code's /branch command. Creates a copy of the current
session's conversation history in a new session, allowing the user to
explore a different approach without losing the original.

Works like 'git checkout -b' for conversations:
- /branch            — auto-generates a title from the parent session
- /branch my-idea    — uses a custom title
- /fork              — alias for /branch

Implementation:
- CLI: _handle_branch_command() in cli.py
- Gateway: _handle_branch_command() in gateway/run.py
- CommandDef with 'fork' alias in commands.py
- Uses existing parent_session_id field in session DB
- Uses get_next_title_in_lineage() for auto-numbered branches
- 14 tests covering session creation, history copy, parent links,
  title generation, edge cases, and agent sync

* fix: clear ghost status-bar lines on terminal resize

When the terminal shrinks (e.g. un-maximize), the emulator reflows
previously full-width rows (status bar, input rules) into multiple
narrower rows. prompt_toolkit's _on_resize only cursor_up()s by the
stored layout height, missing the extra rows from reflow — leaving
ghost duplicates of the status bar visible.

Fix: monkey-patch Application._on_resize to detect width shrinks,
calculate the extra rows created by reflow, and inflate the renderer's
cursor_pos.y so the erase moves up far enough to clear ghosts.

hermes_cli/commands.py

@@ -57,6 +57,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("undo", "Remove the last user/assistant exchange", "Session"),
     CommandDef("title", "Set a title for the current session", "Session",
                args_hint="[name]"),
+    CommandDef("branch", "Branch the current session (explore a different path)", "Session",
+               aliases=("fork",), args_hint="[name]"),
     CommandDef("compress", "Manually compress conversation context", "Session"),
     CommandDef("rollback", "List or restore filesystem checkpoints", "Session",
                args_hint="[number]"),