feat(agent): add durable NOW operations

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• add daemon-owned, bounded NOW operation records with list, status, attach, cancel, and live stdin
• add JSON/NDJSON output, typed errors, diagnostics, Ctrl-C cancellation, and structured process arguments
• document the AI-oriented NOW operation workflow and retention/backpressure limits

Validation

• cargo +stable test -p ironrdp-agent --lib
• cargo +stable clippy -p ironrdp-agent --all-targets --no-deps -- -D warnings
• cargo +stable test -p ironrdp-agent --test remote_exit
• cargo +stable test -p ironrdp-testsuite-extra --test integration_tests_extra agent