[DOC]: agent release notes omit the v7 log-file differences: stderr merged into .log at 0600, interrupt log location, leftover .log.err files

[rice-riley]

Summary

agent/RELEASE_NOTES.md records the [out]/[err] prefix change for v7 but not three other log-file differences from agent/v6.x that showed up when running both agents on the same node (#685 release testing on vr200):

1. stderr is merged into the step's .log, and log files are mode 0600. The Python agent wrote stdout to <step>-<timestamp>.log (0644) and stderr to a sibling <step>-<timestamp>.log.err. Anyone collecting host logs as a non-root user, or expecting a separate stderr file, will notice.
2. Interrupt logs live somewhere else. Go writes <SKYHOOK_LOG_DIR>/<package name from the NodeWright>/<version>/interrupts/<type>-<timestamp>.log; Python wrote <image package name>/<version>/interrupts/<operation index>-<timestamp>.log. For a svc package built from the shellscript image that is svc/1.1.1/interrupts/service_restart-...log versus shellscript/1.1.1/interrupts/0-...log.
3. Neither agent reaps the zero-byte .log.err files a v6.x node already carries. Python's reaper globbed *.log, Go's parses the <step>-<timestamp>.log shape; both skip .log.err, so they accumulate until removed by hand.

Fix

Three bullets under Behavior Changes in agent/RELEASE_NOTES.md (linked PR). No code change proposed: the merged log is a reasonable choice and the location change is cosmetic; the notes just need to say so before v7.0.0 ships.

[rice-riley]

(see above)