feat(go): report retained uptime to local and cloud

[samuelbles07]

Summary

Report retained whole-minute uptime through both Local Server measures and cloud measurement POSTs. The shared uptime utility lives in airgradient-common, advances across deep sleep, and starts a new session after non-deep-sleep resets.

Changes

• Add retained monotonic time support to the RTOS abstraction
• Add shared retained uptime calculation in airgradient-common
• Report dynamic uptime through Local Server and cloud measurement payloads
• Require HTTP measurement POST callers to provide the boot value
• Update host integration coverage and service documentation

Validation

• clang-format checks passed
• Markdown pre-commit checks passed
• Host syntax check passed for the shared uptime source
• Full host tests and Go/reference firmware builds passed before the final common-source relocation; final CI is pending
• Hardware validation remains pending for deep-sleep retention and reset boundaries