ai: refactor multiple skills -> single skill (#6156)

This CL changes the approach of ahving N skills, each of which is
doing a specific thing with Perfetto to instead be a single skill
which internally has a tree of references/workflows guiding the
agent on how to work with traces.

The reason for this change is that we've found it's quite painful
for folks to juggle N skills and it's easy to accidentally only
install partial set of skills. This is problematic as then the
agent can get very confused and go in circles.

If we instead have everything packaged in a single skill and
then give strong guidance on how to "route" itself, it makes it a
lot easier. There's also nice advantages when it comes to
things like sharing common scripts and fragments - we can reference
other files using relative paths in a way which would be very
difficult if we were instead using N skills and couldn't count on
all skills necessarily being installed.