Context tree: relocate from top panel to context side panel

[jeonghun-jj-lee]

Summary

The context tree (the interactive graph showing what the agent holds in context) occupies
a persistent 192px top panel on every session. It adds no actionable information at a
glance and reads as visual noise — it should be relocated to the context side panel
(the drawer opened by the progress-circle ring button in the session header) where it
becomes on-demand rather than always-present.

Important

Problem: The context tree's persistent top-panel placement consumes prime chat space
without aiding pulse-design decisions — it's orientation at the cost of focus.

Approach: Move the graph canvas into the context side panel (top section, above
stats), delete the top panel, return 192px to the chat timeline. On-demand instead of
always-on.

Scope: UI relocation only — the engine (context-tree-engine.ts), data derivation
(context-tree-data.ts), interactivity (click-to-open, keyboard nav), and hover-glance
are unchanged. No new features.

Assumption: The narrower side-panel width (~320–400px) works with the existing
force-settled layout; the engine's resize() and auto-fit camera already adapt.

Acceptance Criteria

• The context tree canvas renders at the top of SessionContextTab (the context
  side panel), 192px height, full panel width.
• The top panel (ContextTreePanel in message-timeline.tsx) is deleted — the chat
  timeline reclaims the space.
• sessionHasContextItems gates the graph section inside the panel (no empty canvas).
• Node clicks still open files: project files → session tab, vault files → Vault panel.
• amicode:brain-hover events from tool rows highlight the corresponding node when
  the context panel is open.
• The engine pause()s when the panel is closed; resume()s on open.
• The localStorage key amicode-context-tree-open (the old collapse toggle) is
  removed.
• Keyboard navigation (arrows, Enter/Space to open) works on the canvas in the panel.
• ADR 0004 recorded; CONTEXT.md "Context tree" entry updated.
• Existing context-tree-engine.test.ts and context-tree-data.test.ts still pass.

Key Decisions

Decision	Rationale
Top of the panel, not a separate tab	The graph is the qualitative counterpart to the stats — same "what's in context" question, visual vs. quantitative
No inner collapse	The panel itself is the toggle; an extra chevron is redundant
192px fixed height	Matches the prior panel; the engine adapts width via ResizeObserver
Supersedes ADR 0003 positioning only	Interactivity, data model, and vault browsability are unchanged

Constraints & Invariants

• context-tree-engine.ts and context-tree-data.ts must not be modified (engine is
  container-size-aware, no layout rework needed).
• The amicode:brain-hover custom event contract is unchanged (emitter stays in
  message-part.tsx; listener moves with the canvas).
• Vault node locked/unlocked status (fail-closed browsability per ADR 0003) is preserved.

Prior Art

• ADR 0003 (the design this supersedes — positioned the graph in the top panel)
• ADR 0002 (brain-as-background — parked direction, not pursued)

Source

Design brainstormed in Amicode session 2026-08-08; resolved via grill-with-docs.
ADR 0004 records the decision (separate PR).