feat: support custom ACP agents

[IchenDEV]

Outcome

Allow users to register, edit, enable, and remove local ACP-over-stdio Agents from Settings while reusing the existing provider registry, Engine, permissions, and session flow.

Change record

• Change: docs/sdlc/changes/2026-09-21-custom-acp-agents/intent.md

The linked change directory owns scope, authorization, acceptance, evidence, and residual risk.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected.
• Required local and independent remote checks passed; see the linked verification record.
• Merge authorization: existing explicit user request pr & merge on 2026-09-21
• Release authorization: not requested