add etm-trace skill: unattended J-Trace ETM capture and analysis

Headless instruction-trace capture on any TRACE_ETM-capable board via
Ozone's automation socket (port 19201, xvfb): etm_capture.py generates a
throwaway project from the board's committed ozone reference (device, TIF,
width, timing, hooks, JLinkScript inheritance), drives the session, and
exports profile/coverage; etm_profile.py renders hot functions, coverage,
per-line/instruction counters and ISR timing from the itrace stream.
SKILL.md carries rig discipline, capture recipes, a new-board bring-up
ladder and troubleshooting; boards.md is the per-board reference (config
table + caveats) for all validated boards.

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