feat(skills): add obsidian-nav — wiki page navigation primitives

[lg320531124]

What changed

Adds a new obsidian-nav skill that provides three navigation primitives for wiki pages:

1. Folded TOC callout — ## 目录 heading as anchor + [!note]+ 📑 目录 callout with clickable section links
2. Per-section return links — [[#目录|↑ 返回目录]] at the end of every ## and ### section
3. Cross-page navigation footer — 📚 导航 block linking sibling pages in the same domain

Why

Wiki pages generated by wiki-ingest currently have no internal navigation. For pages with 5+ sections, readers must scroll linearly — there is no way to jump between sections, return to the table of contents, or traverse to sibling pages without going back to index.md.

The existing obsidian-markdown skill covers Obsidian syntax (wikilinks, callouts, embeds), but does not cover navigation patterns (TOC callout structure, return link conventions, cross-page footers). This skill fills that gap at the orchestration level.

How it works

TOC callout pattern

Key insight: callout titles don't generate anchor targets in Obsidian. The ## 目录 heading is the actual anchor; the callout provides the visual folded TOC.

## 目录

> [!note]+ 📑 目录
> [[index|← 返回知识库]]
> - [[#1. Main Section]]
>   - [[#1.1 Subsection]]
> - [[#2. Another Section]]

Return links

Every ## and ### section gets [[#目录|↑ 返回目录]] at its end. No emoji in the anchor — [[#目录]] resolves, [[#📑 目录]] does not.

Cross-page footer

> 📚 导航: [[index|📑 知识库]] · [[concept-a|概念A]] · [[concept-b|概念B]]

Integration points

• wiki-ingest: Apply after page creation when page has 3+ ## sections
• wiki-lint: Add WARN for pages missing navigation (no ## 目录, missing return links, no footer)

Testing

This is a new skill with no code changes to existing files. The skill is declarative (SKILL.md only) — no scripts, no hooks, no network calls. Hermetic by construction.

Existing test suite (make test) is unaffected — no existing files modified.

Checklist

• Read every file touched (1 new file only)
• Smallest unit that works (single SKILL.md, no speculative abstraction)
• No network calls
• No path construction from user input
• Failure modes documented (What NOT to Do table)
• Follows project naming convention (obsidian- prefix like obsidian-markdown, obsidian-bases)
• Includes 10-principle thinking mapping