docs: separate device-specific implementation notes

[MaxRink]

Summary

• Move named-device protocols, firmware quirks, simulator behavior, and hardware troubleshooting into doc/device-notes/.
• Keep cross-device transport, connection, API, storage, testing, and controller architecture in the existing general docs.
• Add an index and update repository guidance and all affected references.
• Document the DE1 active-GHC restriction: remote starts only arm the white ready indication for UL compliance, while remote stops remain supported.
• Add examples/plugins/AGENTS.md guidance for BLE create() versus sensor registration, per-instance ownership, session fencing, settings routing, and staged verification. Initial single-device drivers require zero/one-device, reconnect, and quota rejection checks; primary/auxiliary checks apply when those capabilities are introduced.
• Archive the whole-series design rationale in doc/plans/archive/skale-grinder-feature-series/design-rationale.md.
• Route implementation details to the independently reviewable concurrent BLE quota, scale metadata, guarded actions, Skale driver, merged SteamSequencer fix, E64 sensor, generic native settings, and sb-dev arguments drafts.

The archived rationale now explicitly delivers #846 first on #844 and primary-only #845, then adds concurrent Skales with #843/#859 in a separate follow-up. Generic settings remain a later independent step.

Linked Issue

N/A - repository documentation maintenance.

Verification

• Combined Linux Flutter gate for the current publication stack: 4,315 passed, 1 skipped, 0 failed; analyzer clean (48.0s). Source was unchanged during the gate.

• Combined simulated runtime checks passed for the current stack; no production fixes were needed during runtime validation.

Impact

Documentation-only guidance and rationale archive. No runtime, API, compatibility, migration, or security impact.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.