feat(runtime-core): forward live sidecar stderr via AGENTOS_SIDECAR_STDERR

[NathanFlurry]

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[NathanFlurry]

Stack for rivet-dev/agentos

• #1815 docs: document sidecar stderr forwarding and host-backed fil …
• #1814 fix(native-sidecar): stop stranding child process.fd_read on …
• #1813 fix(native-sidecar): close maxFilesystemBytes gaps on host-b …
• #1812 docs(load-testing): correct the ledger after adversarial rev …
• #1811 fix(native-sidecar): close remaining guest-reachable fatal p …
• #1810 fix(runtime-core): bound retained sidecar stderr and always …
• #1809 fix(native-sidecar): always answer sync-RPC waiters on handl …
• #1804 fix(native-sidecar): tolerate guest filesystem-quota errors …
• #1806 fix(native-sidecar): stop routing mapped host fds into the k …
• #1807 fix(native-sidecar): enforce maxFilesystemBytes on mapped ho …
• #1802 fix(native-sidecar): keep guest-triggered errors from termin …
• #1801 feat(runtime-core): forward live sidecar stderr via AGENTOS_ … 👈
• #1805 fix(runtime-core): handle stdin error events in StdioFrameTr …
• #1803 test(load): add bounded Docker load-testing harness and issu …
• main

Get stack: forklift get 1801
Push local edits: forklift submit
Merge when ready: forklift merge 1801

_{change ksksyyuv}