feat: Document engine — CodeMirror + uteke doc integration

[ajianaz]

Scope

Uteke v0.3.x document engine integration ke CorIn:

• CodeMirror 6 markdown editor (already in package.json deps)
• Doc list view (sidebar or new tab)
• Doc search bar with chunk-level semantic search
• Doc to memory linking (show related memories)
• Export to .md file

Priority: MEDIUM | Phase: v0.2.0-v0.3.0

Part of original #55 (Agent Integration — item 4)

[ajianaz]

Update: API Client Prerequisite

Document engine UI (this issue) depends on uteke_client.rs doc API methods. Split into:

• feat: Complete uteke_client.rs — add missing doc, graph, tags, pin, timeline methods #115 — Complete uteke_client.rs (doc + graph + tags + pin + timeline methods) — HIGH priority, must land first
• feat: Drop uteke-core dependency — full HTTP-only architecture #117 — Drop uteke-core dependency (requires graph mutation HTTP endpoints from feat: Complete uteke_client.rs — add missing doc, graph, tags, pin, timeline methods #115)

Kanban tasks created on `codecora` board:

• `t_3e955067`: doc API methods (parent of this UI work)
• `t_89cec7f4`: document tree UI (child — this issue's implementation)
• `t_a55855db`: drop uteke-core (child of graph API work)

This issue should focus on UI layer only after #115 is merged.

[ajianaz]

Superseded by #137 (full reimplementation). Document engine shipped in v0.3.0 + tree overhaul in v0.3.1. Closing as completed.