feat(read): add tail mode for file reads

[wsp1911]

Summary

Add tail support to the Read tool so models can read the last N lines of log/output files, while keeping offset for normal forward reads. Update file read presentation and the Read card UI to handle tail reads and preserve legacy start_line display for older persisted data.

Fixes #

Type and Areas

Type:

Feature

Areas:

Rust core, execution primitives, web UI

Motivation / Impact

This gives the model a direct way to poll the end of growing files, which is useful for command output and logs. offset remains the standard forward-read parameter, and tail=true now covers tail-follow workflows without overloading line offsets.

Verification

• pnpm run type-check:web
• cargo test -p bitfun-core read_ -- --nocapture
• cargo test -p tool-runtime read_file_tail -- --nocapture

Reviewer Notes

tail=true is mutually exclusive with offset. The result payload still reports the actual starting line so existing read-state and edit guardrails continue to work.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.