feat(server): add Dirac as a built-in ACP provider

[mhingston]

Summary

Adds Dirac as a built-in provider in Paseo. Dirac is an autonomous coding agent CLI by Dirac Delta Labs with file editing, command execution, browser automation, and MCP support. It supports the Agent Client Protocol (ACP) via the --acp flag.

Closes #590

Changes

• provider-manifest.ts: Registered Dirac in AGENT_PROVIDER_DEFINITIONS with standard ACP modes (Agent, Plan, Autopilot)
• provider-registry.ts: Added Dirac factory using GenericACPAgentClient with ["dirac", "--acp"]
• persisted-config.ts: Added "dirac" to BUILTIN_PROVIDER_IDS for config validation
• provider-launch-config.test.ts: Updated test's builtin provider list
• provider-registry.test.ts: Updated test to account for Dirac's GenericACPAgentClient constructor call
• CLI help text: Updated to include dirac in provider argument hints
• CUSTOM-PROVIDERS.md: Documented Dirac as both a built-in provider and as a custom ACP provider example for users who want to configure it manually

Testing

• Unit tests: All 60 tests pass across provider-launch-config.test.ts, provider-registry.test.ts, and persisted-config.test.ts
• Formatting: Passes oxfmt --check
• Linting: Passes oxlint with 0 warnings, 0 errors
• Note: The full monorepo typecheck has pre-existing failures (missing @getpaseo/highlight build artifacts, implicit any types in unrelated files) which are not introduced by this change

How it works

Dirac is registered as a built-in ACP provider — just like Copilot is a built-in ACP provider. Paseo spawns it with dirac --acp and communicates via the ACP protocol over stdio. Models and modes are discovered dynamically at runtime from the Dirac process. Users can also override or customize it via config.json using the usual custom provider mechanism.