browser: ignore inherited GIO launch markers

[hongnoul]

Summary

• require GIO_LAUNCHED_DESKTOP_FILE_PID to identify the current process before treating a GIO marker as user activation
• keep explicit JCODE_OPEN_ORIGIN=user behavior unchanged
• add inherited/missing/invalid PID regression coverage
• update the agent guide to the merged half-width default

Follow-up to #59.

Validation

• cargo fmt --all --check
• cargo test -p hwatu (83 passed)
• cargo check --workspace --locked
• git diff --check