feat: show conversation file size in the search results

[brtkwr]

What

Adds a SIZE column to the search results so you can see which conversations are bloated and worth pruning (groundwork for the prune feature).

• Stores the .jsonl file size on Conversation (the file is already os.Stat'd during parse, so no extra I/O).
• Renders it human-readable in a 6-wide column: 512B, 2KB, 75MB, 1.2GB.
• TOPIC shrinks 40→34 columns to keep the table from getting too wide.

  DATE              PROJECT               TOPIC                       MSGS  HITS    SIZE
  2024-01-08 15:04  project-name          ✎ Refactor auth flow          42     3   1.2GB
> 2024-01-08 14:30  selected              This one is selected          28     1    12MB

Note / follow-up

The default --max-size=1024 skips files over 1GB, so the very largest conversations (the prime prune targets) only show with --all or a higher --max-size. The upcoming ccs prune CLI will scan all files directly regardless of this filter; I can also add a README note about using --all to surface the giants.

Tests

• TestFormatBytes - formatting across B/KB/MB/GB and that output never exceeds the 6-wide column.
• TestFormatListItemShowsSize - the size renders in a row.
• go test -cover = 69.4%.