fix(cli): restore read tool streaming

[marius-kilocode]

The OpenCode v1.14.51 integration in #11031 resolved conflicts in the Read tool by replacing the streaming path restored in #10077 with an fs.readFile whole-file decode. That made ordinary text reads buffer complete files before applying offsets, limits, or the 50 KB output cap, and left the streaming helper unused. In the same path, zero limits could produce non-advancing pagination, while UTF-16 truncation could split surrogate pairs and produce provider-rejected tool output (#10467).

This restores UTF-8 reads through AppFileSystem.stream and uses buffered encoding detection only as a fallback when bytes are not valid UTF-8. Stream teardown follows output caps, Effect interruption, and session cancellation. Explicit offsets and limits retain their current behavior, zero limits clamp to one item, and both line-level and compacted tool output preserve Unicode surrogate pairs.

The contract remains explicit: an omitted offset still means line 1, and the tool does not maintain an implicit per-file cursor. Existing total-line reporting semantics are also unchanged, so line-limited reads can continue scanning to EOF even though file contents are no longer buffered in memory.

Fixes #10467