docs: Write Concepts documentation (EN)

[100gle]

Parent Issue

Part of #2 — Phase 3: Documentation Content (EN)

Objective

Create the Concepts section — deep explanations of Bub's core ideas.

Source Material

• docs/architecture.md: Turn lifecycle, hook priority, error behavior, boundaries
• docs/features.md: Hook-first runtime, tape context, builtin batteries
• src/bub/framework.py: BubFramework class, process_inbound() flow
• src/bub/hookspecs.py: All 13 hook definitions
• src/bub/hook_runtime.py: HookRuntime execution logic
• src/bub/builtin/context.py: Tape context building
• docs/posts/2026-03-01-bub-socialized-evaluation-and-agent-partnership.md: Philosophy

Files to Create

1. src/content/docs/en/concepts/architecture.mdx

• Core components overview (BubFramework, HookRuntime, BubHookSpecs, Agent, ChannelManager)
• Component interaction diagram
• Boundaries and design principles
• Envelope is intentionally weakly typed
• No centralized state schema

2. src/content/docs/en/concepts/turn-lifecycle.mdx

• Step-by-step walkthrough of process_inbound()
• Diagram: resolve_session → load_state → build_prompt → run_model → save_state → render_outbound → dispatch_outbound
• Error handling: on_error observers, exception propagation
• Fallback behaviors (no model output, no render_outbound)
• Stream events during model execution

3. src/content/docs/en/concepts/hooks.mdx

• What are hooks (pluggy foundation)
• Hook types: call_first vs call_many
• All 13 hooks listed with signatures and semantics
• Priority: builtin first, external plugins override
• Sync vs async rules
• Signature matching (declare only needed params)

4. src/content/docs/en/concepts/tape-context.mdx

• What is tape? (Republic, append-only facts)
• Anchors and phase transitions
• Context reconstruction on demand vs session accumulation
• Handoff pattern
• Token usage awareness
• Tape forking for sub-agents
• Comparison: tape vs traditional chat history

Acceptance Criteria

• All four files created with frontmatter and sidebar ordering
• Diagrams described (can be text-based, SVG added in Phase 5)
• Hook signatures accurate against src/bub/hookspecs.py
• Turn lifecycle matches src/bub/framework.py implementation
• Clear, educational tone suitable for developers new to Bub

[100gle]

✅ Completed — en/concepts/index.mdx and en/concepts/architecture.mdx migrated from docs/architecture.md. Covers core components, turn lifecycle, hook priority, error behavior, builtin runtime, channel streaming, and boundaries. zh-cn mirrors created.