feat(fetch,llm): DoS hardening via response caps + glob validation (P2)

Response body caps:
- webclaw-fetch::Response::from_wreq now rejects bodies over 50 MB. Checks
  Content-Length up front (before the allocation) and the actual
  .bytes() length after (belt-and-braces against lying upstreams).
  Previously the HTML -> markdown conversion downstream could allocate
  multiple String copies per page; a 100 MB page would OOM the process.
- webclaw-llm providers (anthropic/openai/ollama) share a new
  response_json_capped helper with a 5 MB cap. Protects against a
  malicious or runaway provider response exhausting memory.

Crawler frontier cap: after each BFS depth level the frontier is
truncated to max(max_pages * 10, 100) entries, keeping the most
recently discovered links. Dense pages (tag clouds, search results)
used to push the frontier into the tens of thousands even after
max_pages halted new fetches.

Glob pattern validation: user-supplied include_patterns /
exclude_patterns are rejected at Crawler::new if they contain more
than 4 `**` wildcards or exceed 1024 chars. The backtracking matcher
degrades exponentially on deeply-nested `**` against long paths.

Cleanup:
- Removed blanket #![allow(dead_code)] from webclaw-cli/src/main.rs;
  no warnings surfaced, the suppression was obsolete.
- core/.gitignore: replaced overbroad *.json with specific local-
  artifact patterns (previous rule would have swallowed package.json,
  components.json, .smithery/*.json).

Tests: +4 validate_glob tests. Full workspace test: 283 passed
(webclaw-core + webclaw-fetch + webclaw-llm).

Version: 0.3.15 -> 0.3.16
CHANGELOG updated.

Refs: docs/AUDIT-2026-04-16.md (P2 section)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,11 @@
 target/
 .DS_Store
 .env
+.env.*
 proxies.txt
 .claude/skills/
-*.json
+# Scratch / local artifacts (previously covered by overbroad `*.json`,
+# which would have also swallowed package.json, components.json,
+# .smithery/*.json if they were ever modified).
+*.local.json
+local-test-results.json