Fix agent process leaks and unbounded output memory

[Bakobiibizo]

What changed

• stream full stdout/stderr to disk while retaining only the configured bounded summary window in memory
• run foreground commands in isolated process groups and forward termination signals
• adopt, terminate, and reap descendants on Linux, including helpers that create a new session/process group
• replace the detached shell wrapper with an internal supervised runner using the same lifecycle guarantees
• bump the patch version to 0.1.1 and document the lifecycle contract

Validation

• cargo test --all-targets
• cargo clippy --all-targets --all-features -- -D warnings
• foreground termination regression with a descendant in a new session
• detached completion regression with a descendant in a new session
• 64 MiB output smoke: 3.8 MiB max RSS, complete 67,108,873-byte raw log, zero zombies afterward