Add session tree navigation commands

[piercebrookins]

Summary

Adds persistent, pi-inspired conversation branching through /tree, /fork, and /clone.

Why

A linear message list cannot preserve an abandoned branch after rewinding. The first implementation rendered tree glyphs beside that linear list, used process-local labels, and destructively discarded history. This revision stores a real append-only conversation graph per autosave session and reconciles the active pydantic-ai history into it.

What changed

• Added an atomic persistent SessionTree store under the autosave directory, with stable node IDs, parent links, labels, timestamps, active-leaf state, and message payloads.
• Reconciles each live linear history into the graph, preserving sibling branches after rewind and subsequent turns.
• /tree renders actual branch depth/connectors and distinguishes the active leaf/path.
• Selecting a user message restores its parent and returns the original prompt for editing; selecting another message restores through that node.
• Added incremental text search plus default, no-tools, user-only, labeled-only, and all-message filters.
• Label and display settings are preserved through selector navigation; labels persist across process restarts.
• /tree summary now summarizes only the abandoned tail after the common ancestor and attaches that context at the destination. Failures leave active history untouched.
• /fork uses the same tree selector in user-only mode rather than an unrelated numbered text menu.
• /clone saves the active branch as a separate autosave artifact without rotating or mutating the current session identity.
• Added regression coverage for branch creation, stable duplicate handling, persistence, labels, active paths, filtering/search, summary boundaries, user-message rewind behavior, /fork, and /clone.

Validation

• uv run ruff check .
• uv run pytest tests/plugins/test_session_tree.py -q --no-cov (14 passed)
• git diff --check
• Full suite: 10,365 passed, 82 skipped, 1 xpassed; 8 unrelated failures remain in pre-existing MCP/live-provider tests (test_builder_autostart_mcp, test_add_model_menu_coverage, and live compaction/provider tests).