fix(core): remove file-promotion + slugify paths + clean dev output + full watch rebuild

[sinsombat]

Background

This PR follows #131. Four related bugs were found while investigating that issue.

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Fix 1 — Remove implicit file-promotion (auto-router.ts)

When a folder had .md files but no index.md, the first file was promoted to the folder URL, silently dropping its filename. This caused inconsistent URLs (files in folders with index.md kept their filename; files without did not) and made same-named files in sibling folders indistinguishable. Removed the promotion block. Every file now always generates a URL that includes its own filename.

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Fix 2 — Slugify path segments (auto-router.ts + generator.ts)

Problem

Files and folders whose names contain spaces or URL-unsafe characters produced 404 in both dev and production:

docs/SA/my feature/page one.md
  nav link → ./SA/my feature/page one/   (literal space)
  browser  → /SA/my%20feature/page%20one/ (%20-encodes the space)
  server   → tries SA/my%20feature/ on disk  → not found → 404

normalizeInternalHref adds a trailing slash but does not slugify segments. The raw OS filename was used verbatim in both the nav href and the site/ output path.

Fix

Added a slugifySegment helper and applied it in both places:

• auto-router.ts — when building each relPath segment from fs.readdirSync
• generator.ts — when building htmlOutputPath from relativePath

Both sides use identical logic so the nav URL and the output file path always agree:

docs/SA/my feature/page one.md
  nav: /SA/my-feature/page-one/   ✓  200
  out: site/SA/my-feature/page-one/index.html  ✓

Slugification rules: spaces → -, [^a-zA-Z0-9\-_.~] → -, consecutive hyphens collapsed, leading/trailing hyphens stripped. Files with already-safe names are unaffected.

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Fix 3 — Clean output dir before initial dev build (dev.ts)

After any change to auto-router URL structure (e.g. applying this PR), old HTML files built under the previous URL structure remain in site/. New nav links point to new URLs but new files may not exist yet — causing 404 on nav click. Added fs.remove(rootOutputDir) before the initial dev build so every docmd dev session starts clean.

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Fix 4 — Full rebuild on file watch (dev.ts)

The file watcher called buildSite with targetFiles: [filePath], regenerating only the modified page. Every other page kept its old nav HTML. Adding a new .md file while the dev server was running meant the link never appeared in the sidebar of existing pages. Removed targetFiles from the watcher rebuild so every change triggers a full rebuild (~200–300 ms for typical doc projects).

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Note

With Fix 1, _sourceFile is never set on nav items, so the navDesignatedIndexFiles logic in generator.ts becomes a no-op. Left in place as it may be useful for future features.