Page tool-output blobs by byte windows not line caps

[TheGreatAxios]

Summary

read_file of a tool-output:// URI pages by byte windows so a single huge line is readable in successive slices. Filesystem reads stay line-oriented. Continuation notices name the URI and the next window.

The old 2,000-line cut fails both shapes of a spilled blob: pretty-printed JSON is thousands of lines, minified JSON is one 100k-character line that the pager cannot scroll.

• Blob path uses the 50KB content budget with 2000-character wraps instead of the source-file line cap.
• A giant one-line blob splits into numbered windows; a follow-up read_file of the cursor URI returns the remainder.
• Ordinary filesystem source files still truncate overlong lines and stop at the default max lines.

Verification

• bun run typecheck, bun run build, and bun run test pass
• bun run check passes
• bun test src/plugins/read-file-guard-plugin.test.ts covers one giant line, a pretty-printed object over the old line cap, and cursor remainder

Fixes CL-7620

[linear-code]

CL-7620