feat(session): add sibling and parent session navigation

[hermandavid]

Summary

Adds two new session navigation actions for easier browsing of child session hierarchies.

Problem

When browsing a child session (e.g., session B, child of A), navigating to a sibling session (another child of A) required going back to the root session list, selecting parent A, then listing its children again. There was also no way to jump directly to a parent session.

Solution

• select_sibling_session: Opens the session picker filtered to siblings (children of the same parent). Falls back to root sessions with a notification when the active session has no parent.
• select_parent_session: Directly switches to the parent session. Notifies when there is no parent.
• select_child_session is unchanged — it still shows children of the current session, preserving grandchild navigation.

Navigation model

From	Action	Result
Root A	child	Picker: children of A
Child B (parent=A)	sibling	Picker: children of A (siblings of B)
Child B (parent=A)	parent	Switch directly to A
Child B (parent=A)	child	Picker: children of B (unchanged)

New keymaps

Keymap	Action	Description
<leader>oB	select_sibling_session	Select sibling session
<leader>oP	select_parent_session	Go to parent session

[jensenojs]

need it!

[sudo-tee]

This is really great.

Thanks for this