fix(extract): skip _-prefixed directories in walkMarkdownFiles (#202)

[sharziki]

Summary

Fixes #202 — `gbrain extract all --source fs` walked ``-prefixed directories even though the walker already skipped ``-prefixed files, causing extract to count quarantined content that the sync path correctly excluded.

Root cause

`src/commands/extract.ts::walkMarkdownFiles()`:

```ts
for (const entry of readdirSync(d)) {
if (entry.startsWith('.')) continue; // applies to both files + dirs
const full = join(d, entry);
try {
if (lstatSync(full).isDirectory()) {
walk(full); // recurses into pending/
} else if (entry.endsWith('.md') && !entry.startsWith('')) {
files.push({ path: full, relPath: relative(dir, full) });
}
}
}
```

The `!entry.startsWith('_')` check only gates files. The directory branch above it recurses unconditionally, so `pending/originals/foo.md` is walked even when the user has standardized on ``-prefix quarantine namespaces. On the reporter's brain: 61 authoritative pages but "154 pages walked" once the `_pending/` tree was included.

Fix

Hoist the `_`-prefix skip above the `lstatSync` branch so it behaves the same as the `.`-prefix skip — applies to both files and directories.

```ts
if (entry.startsWith('.')) continue;
if (entry.startsWith('_')) continue;
```

Test plan

• New test in `test/extract.test.ts` builds a fixture with `concepts/alpha.md`, `_pending/ambient.md`, `_pending/originals/buried.md`, and a sibling `_skip-me.md`. Expects `walkMarkdownFiles` to return exactly `['concepts/alpha.md']`.
• `bun test test/extract.test.ts` → 17/17 pass.

🤖 Generated with Claude Code