fix: preserve Tistory article bodies

[code-yeongyu]

Summary

Fixes the reader-mode extraction path for Tistory-style posts where Mozilla Readability can choose surrounding blog chrome instead of the article body. The fetcher now also keeps browser-shaped navigation headers without falling back to a bot-like retry identity.

Changes

• Prefer explicit article containers used by Tistory and common blog themes, while stripping related-post/sidebar/comment/ad chrome from the cloned article node.
• Preserve readable line breaks by converting HTML fragments through a DOM pass before falling back to regex cleanup.
• Use undici.request with browser navigation headers, manual redirect handling, bounded body reads, and no Cloudflare challenge retry with pi-webfetch user-agent.
• Add regression tests for Tistory wrappers, newline preservation, navigation headers, Cloudflare no-retry behavior, and redirect-limit body handling.

QA / Evidence

• npx vitest --run test/webfetch.test.ts: 15 tests passed. Artifact: /Users/yeongyu/local-workspaces/senpi/.omo/evidence/webfetch-tistory-worktree-pr/external-verification-after-discard-fix.txt.
• npm run check: tsgo --noEmit && biome check . passed. Artifact: /Users/yeongyu/local-workspaces/senpi/.omo/evidence/webfetch-tistory-worktree-pr/external-verification-after-discard-fix.txt.
• Public Tistory smoke was captured earlier in the ULW evidence set and confirmed the extracted text included article content and excluded another_category. Artifact: /Users/yeongyu/local-workspaces/senpi/.omo/evidence/webfetch-tistory-worktree-pr/external-live-tistory.txt.

Risks

• The explicit article selector path is intentionally conservative: it only wins when the cloned article text is at least 30 characters, then Readability remains the fallback.
• Browser headers may need periodic UA refresh later, but this PR keeps the header shape covered by tests.

Secret safety

No secret-bearing logs, tokens, auth headers, cookies, or credentials are included in the evidence or PR body.

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Summary by cubic

Fixes Tistory reader-mode by preferring real article containers and preserving line breaks, so posts no longer pick sidebar/category chrome. Also switches to browser-shaped fetching via undici.request for more stable loads and safer redirects.

• Bug Fixes
  • Prefer known Tistory/common blog selectors; strip sidebars, comments, ads; fall back to Readability only if needed.
  • Preserve readable line breaks using a DOM pass for text and normalize markdown spacing.
  • Use browser navigation headers with undici.request, manual redirect handling, bounded body reads, and no bot-identity retry on Cloudflare challenges.

^{Written for commit 83266e8. Summary will update on new commits.}