Expose settings read and write over RPC

[m-aebrer]

Summary

The planned web dashboard (#307) includes a settings tab ("model/provider choices and dreb-relevant runtime settings", with explicit validation errors). RPC mode currently offers write-only, per-runtime toggles and no way to read effective settings or persist defaults. This issue adds a typed settings read/write surface over RPC.

Current Behavior

• RPC exposes per-runtime setters only: set_thinking_level, set_steering_mode, set_follow_up_mode, set_auto_compaction, set_auto_retry, set_model.
• There is no RPC command to read current/effective settings (a client cannot even render current steering mode without tracking every write).
• Persistent defaults (SettingsManager: default provider/model, default thinking level, steering/follow-up modes, compaction enabled, retry enabled, etc.) are not writable over RPC — runtime setters do not persist across restarts unless the underlying AgentSession method persists them.

Proposed Behavior

• New RPC command get_settings returning a typed snapshot of dashboard-relevant effective settings: default provider/model, default thinking level, steering mode, follow-up mode, compaction enabled, retry enabled (exact list to be finalized against the UX spec from Design dreb dashboard UX/UI for TUI feature parity #311, but this set is the known-needed core).
• Typed write command(s) for persistent defaults backed by SettingsManager — either individual commands mirroring the existing setter style or a single set_settings with a strictly validated typed payload. No generic string key/value writes.
• Validation failures return explicit RPC errors; no silent fallback or clamping.

Acceptance Criteria

• get_settings command and typed response added to rpc-types.ts and handled in rpc-mode.ts.
• Typed persistent writes for at least: default provider/model, default thinking level, steering mode, follow-up mode, compaction enabled, retry enabled.
• Writes persist via SettingsManager and are reflected in subsequent get_settings reads and fresh runtimes.
• Invalid values produce descriptive errors.
• RpcClient gains corresponding methods.
• Tests cover read, write, persistence, and validation-error paths.
• docs/rpc.md documents the settings surface.

Context

• Parent: Build first-class dreb web dashboard #307 (dashboard implementation)
• Identified as a prerequisite in the assessment on Design dreb dashboard UX/UI for TUI feature parity #311 (dashboard UX design)

Technical Notes

• core/settings-manager.ts already has typed getters/setters for everything listed — this is plumbing plus DTO design, not new settings logic.
• Distinguish clearly between per-runtime state (existing commands, unchanged) and persistent defaults (new). get_settings should make the distinction legible to clients, since the dashboard settings tab will surface both.
• TUI-specific settings (themes, hardware cursor, editor padding, buddy) are out of scope.