Move agent roles and workflows out to agentrc (#3920)

The Claude/Codex agent roles, review workflows and generic USB skills now
come from the user-level agentrc install: pr-babysit, pr-review-validator,
pr-ci-watcher, code-writer, code-verifier, finding-verifier, pvs-studio,
code-audit (was driver-review), usb-sniffer, usb-kernel-debug and the Codex
bridge. What stays is TinyUSB-specific: the build skill as the project's
build contract (check_build.py: scope resolution through ci_select, one
representative board per affected family and USB IP, dependency pins,
shared-cache staleness, coverage verdict), the HIL and validation workflows,
build-doc, make-release, and the CLAUDE.md recipes that invoke the installed
roles. get_deps.py --gen-doc now writes docs/reference/dependencies.rst.