feat(cli): add --max-output-bytes and --mode summary,toc for output-size control

Three additive CLI flags addressing the 50KB persisted-output cap that
trips Claude Code's per-tool-result harness on aggregator front pages
(apnews.com, cnbc.com/markets/, b92.net all >50KB by default):

--max-output-bytes N: truncates final output at N bytes with a clear
'[truncated: M more bytes ...]' footer. N=0 means unlimited (default).
UTF-8 codepoint-boundary safe; also wraps JSON output so truncated
output stays parseable.

--mode summary: returns only the extracted link list (titles + URLs),
no body text. For aggregator front pages where the LLM is going to
drill the individual articles next anyway.

--mode toc: returns H1/H2 outline + first paragraph after each H2.
For long single-article pages.

New flags are orthogonal to -f (json/llm/text). 9 new unit tests in
webclaw-core, total goes 308 -> 317 passing. Smoke-tested on
apnews.com (51713 -> 27404 summary -> 6269 toc -> 8193 capped),
pitchfork.com (42049 -> 379 summary), cnbc.com (56682 -> 16385 capped).

crates/webclaw-core/src/llm/mod.rs

@@ -9,6 +9,12 @@ mod cleanup;
 mod images;
 mod links;
 mod metadata;
+mod output_size;
+
+pub use output_size::{
+    to_json_summary, to_json_toc, to_llm_summary, to_llm_toc, truncate_json_with_wrapper,
+    truncate_with_footer,
+};
 
 use crate::types::ExtractionResult;