fix(shell): stream output losslessly with backpressure

[furgalep]

Summary

• deliver BashSession streaming output incrementally and without truncation
• bound in-process memory with one pending read per stdout and stderr stream, allowing consumer backpressure
• preserve split UTF-8 sequences with incremental decoders
• propagate early consumer closure through ShellTools, interrupt the active command, and resynchronize the persistent shell
• use the existing TruncatingStringIO for bounded buffered run output instead of unbounded byte accumulation

Behavioral contract

• run and run_with_timeout_flag return bounded head/tail output
• run_stream delivers every decoded chunk exactly once and emits one terminal done event
• stdout and stderr ordering is preserved independently
• timeout and natural exit code 124 remain distinguishable
• abandoning a stream does not leak a running command or stale output into the next command

Tests

• 58 focused shell tests pass
• covers large stdout and stderr, incremental delivery, UTF-8 read boundaries, timeout recovery, natural exit 124, cross-loop reuse, and consumer abandonment
• Ruff check and format validation pass

Configuration note

Buffered BashSession output currently retains the existing fixed 30,000-character budget per stream. Wiring this to the agent TruncationConfig capture settings is intentionally left for a separate configuration change.