docs(adr): 0030 optional modules share one contract and load only when turned on

[V3RON]

Proposes ADR 0030 for #591 (optional modules share one contract and load only when turned on). It narrows ADR 0023 §3.

It decides:

• An optional module is one definition.ts and one line in src/modules/registry.ts. Its one name is its capability, its relay target, its doctor module field and, in camelCase, its config key.
• A module turned off is never loaded and reports off. A leftover agent-device daemon is no longer reaped while agent-device is off.
• createWorker(ctx) returns a handle: what the module provides, plus hooks named for the host step they run at (afterSocketClaimed, beforeDeviceReclaim, beforeDrain, beforeDriversDispose). The host bounds them from one table.
• The event bus is for observing only; a step the host waits for is a hook.
• A gateway module gets a fleet context bound to its name; agent-device's fleet port leaves main.ts.
• Each module owns its config; keys stay top-level.
• ModuleMount lives in src/modules, so no module imports src/http.
• agent-device-unavailable becomes module-unavailable with module. The protocol goes from 36 to 37.

Stays open as the bottom of #591's stack until the feature's last task merges.

Status

• ADR drafted at Proposed, index row added
• Spec check findings folded in
• Maintainer accepts the ADR (Accepted — not yet implemented)

Assumptions

None.

Review

Spec review: skipped. Code review: skipped. Claims review: skipped.
Rounds: 0. Tokens: 0M.

The maintainer accepted the ADR directly; no review run was recorded on this PR.

Written by an agent.