fix: batch resolve issues #38–#42

[philoserf]

Summary

• bug: truncateHeadOnly always appends ellipsis even when no content was cut #38 truncateHeadOnly: only append ellipsis when content was actually truncated
• bug: truncateHeadTail emits separator when tail is empty #39 truncateHeadTail: omit \n...\n separator when tail budget is zero
• bug: truncateHeading always appends ellipsis to first paragraph even when not truncated #40 truncateHeading: only append ellipsis to paragraphs exceeding 30 tokens; also fixed body section
• bug: parseMetadataResponse throws on malformed JSON instead of returning null #41 parseMetadataResponse: wrap JSON.parse in try/catch, return null on malformed JSON instead of throwing
• bug: code-fence stripping regex removes triple backticks from entire response #42 Code-fence stripping: try direct JSON match first to preserve embedded triple backticks

Closes #38, closes #39, closes #40, closes #41, closes #42

Test plan

• All 88 existing tests pass
• Updated tests for bug: truncateHeadOnly always appends ellipsis even when no content was cut #38 (no ellipsis when limit >= length)
• Updated test for bug: truncateHeadTail emits separator when tail is empty #39 (no separator when limit is 1)
• New test for bug: truncateHeading always appends ellipsis to first paragraph even when not truncated #40 (short paragraph without spurious ellipsis)
• Changed bug: parseMetadataResponse throws on malformed JSON instead of returning null #41 test from toThrow to toBeNull
• New test for bug: code-fence stripping regex removes triple backticks from entire response #42 (triple backticks inside JSON value preserved)
• bun run validate passes (typecheck + lint + build)

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