feat: add Pi-style /tree conversation navigation

[piercebrookins]

Summary

Add a built-in /tree plugin for navigating and branching the current conversation without changing Code Puppy's existing /fork background-agent command.

Behavior

• renders the complete append-only conversation graph with the active path first
• selecting a user message rewinds before that prompt and restores its text into the idle editor for editing/resubmission
• selecting assistant/tool entries rewinds at that entry
• selecting the current leaf is a no-op
• resubmitting after a rewind preserves the abandoned path as an alternate branch
• persists inactive branches in atomic, session/agent-keyed sidecars
• retains branches across provider history compaction and root rewinds
• supports search, active-path markers, folding, paging, copy, and default/no-tools/user/all filters
• hides tool-call-only assistant nodes in the default and no-tools views
• localizes all new user-facing strings

/fork and /forks are intentionally untouched; their existing background sub-agent semantics and tests remain intact.

Implementation

The feature is plugin-first under code_puppy/plugins/tree/:

• tree_model.py — append-only graph, provider-message extraction, filtering, and Pi-compatible navigation semantics
• tree_menu.py — prompt_toolkit selector and keyboard controls
• tree_storage.py — atomic sidecar persistence
• register_callbacks.py — slash-command/help/lifecycle integration and editor restoration

No command-line command was added to core.

Test plan

• uv run ruff check code_puppy/plugins/tree tests/plugins/test_tree_plugin.py
• uv run ruff format --check code_puppy/plugins/tree tests/plugins/test_tree_plugin.py
• uv run pytest -q tests/plugins/test_tree_plugin.py tests/plugins/test_fork_plugin.py tests/plugins/test_plugin_contributions.py tests/i18n/test_i18n_coverage.py --no-cov
  • 77 passed before the final compaction/tool visibility hardening
• final focused run:
  • 42 passed across /tree, /fork, and i18n suites
• full repository suite:
  • 12,457 passed, 81 skipped, 1 xpassed
  • one unrelated order-dependent config assertion failed (protected_token_count expected 25000, observed 24576)
  • the same test passes in isolation

The /tree suite includes a real POSIX pseudo-terminal smoke test that opens the selector, sends Up + Enter, and verifies the selected user prompt rewinds provider history to its parent.