fix launcher telemetry JSON escaping

[jhontejada95]

Summary

• replace the lossy _msan filter with a POSIX od/awk JSON string encoder
• escape quotes, backslashes, standard escapes, and every remaining 0x00–0x1f control byte while preserving UTF-8
• keep the root, packaged, and Antigravity launcher scripts byte-identical
• add a regression test to the Unix CI matrix

Root cause

The POSIX launcher builds its early PostHog payload by hand. Its sanitizer deleted quotes, backslashes, CR, and LF, but left tabs and other control bytes in JSON strings. Those bytes make the payload invalid, while deleting valid characters also silently changed telemetry values.

Impact

Dynamic launcher properties containing quotes, slashes, whitespace controls, or UTF-8 now remain valid and round-trip without data loss, preventing affected telemetry events from being corrupted or dropped.

Validation

• ./tests/monk-launcher-telemetry-json.sh
• sh -n on all three telemetry scripts and the regression test
• ./tests/start-monk-agent-fastpath.sh
• ./tests/start-monk-agent-readiness-timeout.sh
• git diff --check

Fixes #179