fix(core): grep line previews include trailing newlines, emitting phantom blank lines in model output

Description

rg --json includes the line's trailing newline in data.lines.text (\r\n for CRLF files). packages/core/src/ripgrep.ts passes it through unmodified into Match.text (only applying the 2000-char truncation), so every grep match carries a trailing newline into both the structured tool output and the model-facing text.

In packages/core/src/tool/grep.ts:47, toModelOutput then renders Line ${match.line}: ${match.text} and joins with \n, producing a phantom blank line after every single match — while blank lines are supposed to separate files only (lines 42-44).

Reproduction

printf 'line a\nline b\n' > t.txt
rg --json line t.txt | jq -c 'select(.type=="match") | {line: .data.line_number, text: .data.lines.text}'
# {"line":1,"text":"line a\n"}   <- trailing \n

Rendering through toModelOutput:

Found 2 matches
t.txt:
  Line 1: line a
                    <- phantom blank line
  Line 2: line b
                    <- phantom blank line

Expected

Line previews should not include the trailing newline; blank lines should only appear between file groups.

Suggested fix

Strip the trailing newline where the preview is shaped in ripgrep.ts (same place the existing 2000-char truncation and the surrogate-pair fix live), e.g. .replace(/\r?\n$/, "") before truncation.

I have a fix + tests ready and will open a PR referencing this issue.