File search: can print lines and line numbers

[BackSlasher]

Closes #9399

Two commits:

1. print matched lines, unless --name-only
2. print line numbers if -n

As specced in #9399 by @martinvonz

Testing

• Existing snapshots updated to reflect the new path:line default output.
• --name-only restores path-only output.
• -n / --line-number prefixes matches with 1-based line numbers.
• --name-only combined with -n errors out (clap-enforced).
• Multi-match files emit one line per match; --name-only collapses to one line per file.
• Conflict files: --name-only deduplicates across matching add-sides.
• Conflict files: -n numbers lines within each side independently.

Known issue: Binary files

Because we now emit matched-line contents (not just paths), a search that happens to match bytes in a binary file will dump raw bytes, potentially including terminal control codes, to stdout.

Addressed in the follow-up PR #9742 (mirrors git grep: Binary file <path> matches by default, -a/--text and -I/--no-binary overrides).

Not in scope

• Templated output (--template): the bigger stretch goal on FR: Allow seeing the matched lines for file search command #9399.
• Labelling which conflict side a match came from: existing TODO in the code.
• Streaming the file reader: plain-file path currently uses read_all, which will inflate process memory on big files. Can be moved to streaming but wanted to keep this PR small.
• Concurrent file reads: existing TODO in the code.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

[BackSlasher]

Rebased because drifted from main. Anything I can do to help this land?