docs(baseosmgr): add architecture document

Adds pkg/pillar/docs/baseosmgr.md describing the responsibilities,
pubsub inputs and outputs, internal logical components, control flow
through the five main paths (boot, BaseOsConfig install, content-tree
progress, test-complete commit, reboot-after-failed-update) plus the
force-fallback and retry-update side channels, and a debugging section.
Structured to mirror the existing nim.md and nodeagent.md so the pillar
docs remain consistent across microservices.

Signed-off-by: eriknordmark <erik@zededa.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: eriknordmark