docs: Write Extending, Reference, and Blog sections (EN)

[100gle]

Parent Issue

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

Objective

Create the Extending section (plugin development), Reference section (API tables), and migrate Blog posts.

Source Material

• docs/extension-guide.md: Full extension guide (hooks, tools, skills, patterns, pitfalls)
• src/bub/hookspecs.py: 13 hook specifications
• src/bub/tools.py: @tool decorator, REGISTRY
• src/bub/builtin/tools.py: All builtin tool implementations
• src/bub/builtin/settings.py: AgentSettings model
• env.example: All environment variables
• docs/posts/2026-03-01-bub-socialized-evaluation-and-agent-partnership.md
• docs/posts/2025-07-16-baby-bub-bootstrap-milestone.md

Files to Create

Extending Section

1. src/content/docs/en/extending/plugins.mdx

• Plugin basics: @hookimpl, class shape
• Entry-point registration ([project.entry-points."bub"])
• Plugin loading order and priority
• Minimal end-to-end EchoPlugin example
• Testing with bub hooks

2. src/content/docs/en/extending/custom-hooks.mdx

• Override vs observe pattern
• Stream wrapping (listen to parent stream)
• subset_hook_caller() pattern
• Sync vs async implementation rules
• Signature matching
• Common pitfalls (stale hook names, missing imports, async in sync paths)

3. src/content/docs/en/extending/custom-channels.mdx

• Channel ABC (src/bub/channels/base.py)
• Implement: name, enabled, start, stop, send, on_event
• Register via provide_channels hook
• Debounce support (needs_debounce)
• Example: skeleton Discord adapter

4. src/content/docs/en/extending/custom-tools.mdx

• @tool decorator usage
• Tool registration (import-time side effect)
• Name resolution (dots vs underscores)
• Tool availability filtering
• Example: custom tool implementation

5. src/content/docs/en/extending/custom-skills.mdx

• SKILL.md anatomy (frontmatter + body)
• Validation rules (name pattern, lengths, metadata)
• Directory structure (scripts/, references/, assets/)
• Packaging skills in extension packages
• Progressive disclosure pattern

Reference Section

6. src/content/docs/en/reference/hook-specs.mdx

• Table of all 13 hooks with: name, type (call_first/call_many), parameters, return type, description
• Extracted directly from src/bub/hookspecs.py

7. src/content/docs/en/reference/builtin-tools.mdx

• Table of all 15+ tools with: name, category, parameters, description
• Grouped by: Shell & FS, Tape Management, Skills & Web, Meta

8. src/content/docs/en/reference/configuration.mdx

• Complete env var table: variable, default, description
• Grouped by: Core, Channel Manager, Telegram, Advanced

9. src/content/docs/en/reference/cli-commands.mdx

• All commands with full syntax, options, examples
• Hidden commands (bub hooks)

Blog Section

10. src/content/docs/en/blog/socialized-evaluation.mdx

• Migrate from docs/posts/2026-03-01-bub-socialized-evaluation-and-agent-partnership.md
• Preserve content, add MDX frontmatter

11. src/content/docs/en/blog/bootstrap-milestone.mdx

• Migrate from docs/posts/2025-07-16-baby-bub-bootstrap-milestone.md
• Preserve content, add MDX frontmatter

Acceptance Criteria

• All 11 files created with correct frontmatter and sidebar ordering
• Extension guide content enhanced with more code examples
• Reference tables accurate against current source code
• Blog posts preserved faithfully
• All internal cross-references link correctly

[100gle]

✅ Completed — en/extending/{plugins,hooks}.mdx split from docs/extension-guide.md (plugins = import/entry-points/tools/skills; hooks = execution semantics/priority/sync-async/examples/pitfalls). en/blog/socialized-evaluation.mdx replaced with full essay. en/blog/bootstrap-milestone.mdx added. en/reference/index.mdx placeholder created. Sidebar updated to use autogenerate per directory. zh-cn mirrors for all.