feat(opencode): hardcode agent to 'build' and remove selection

[aaight]

Summary

• Hardcoded OpenCode agent to "build" — Removed auto/build/plan selector, always uses build agent
• Removed agent field from OpenCode schema, settings resolver, and catalog definition
• Updated UI — OpenCode settings now only show "Web Search" toggle, agent selection removed
• No DB migration needed — Existing stored settings with agent key are safely stripped by Zod's strip mode

Changes Made

Step 1: Removed agent from OpenCodeSettingsSchema

• File: src/config/engineSettings.ts
• Removed the agent: z.enum(['auto', 'build', 'plan']).optional() property from the schema

Step 2: Updated ResolvedOpenCodeSettings interface and resolver

• File: src/backends/opencode/settings.ts
• Removed agent from the ResolvedOpenCodeSettings interface (now only requires webSearch)
• Updated resolveOpenCodeSettings() to return only { webSearch } instead of { agent, webSearch }

Step 3: Removed resolveOpenCodeAgent function and hardcoded 'build'

• File: src/backends/opencode/index.ts
• Deleted the resolveOpenCodeAgent() function entirely
• In execute() method: replaced dynamic agent resolution with hardcoded const agent = 'build' as const

Step 4: Removed agent field from engine catalog

• File: src/backends/catalog.ts
• Removed the entire agent select field from OPENCODE_ENGINE_DEFINITION.settings.fields
• Updated description to remove reference to agent selection

Step 5: Updated unit tests

• File: tests/unit/backends/opencode.test.ts
• Removed resolveOpenCodeAgent import and its entire test suite
• Updated resolveOpenCodeSettings tests to expect only { webSearch }

Testing

✅ All 4,573 unit tests passing
✅ Typecheck: zero errors
✅ Linting: zero errors

Impact

• Behavior change: OpenCode always uses the "build" agent (previously could be auto/build/plan)
• Dashboard UI: OpenCode settings panel now only shows "Web Search" toggle
• Backward compatibility: Existing DB records with agent field will be safely ignored (Zod strip mode)
• No API changes: Settings are internal to OpenCode engine configuration

https://trello.com/c/eqoHGk2R/296-remove-agent-selection-in-opencode-and-hardcode-to-build

[nhopeatall]

Summary

Clean, well-scoped removal of the OpenCode agent selector. The change is correct and complete across the main code paths — schema, settings resolver, engine execution, catalog, and primary tests are all updated consistently. One stale test reference was missed.

Should Fix

• tests/unit/config/schema.test.ts:117-135 — The test "accepts opencode engine settings on project config" still sets agent: 'plan' in the input (line 126) and asserts result.engineSettings?.opencode?.agent equals 'plan' (line 133). This passes at runtime because EngineSettingsSchema's transform preserves raw key-value pairs via normalizeEngineSettings, but it's testing behavior for a removed field. This test should be updated to remove the agent references and only assert on webSearch, matching the cleanup done everywhere else in this PR.

Notes

• buildConfig still configures both build and plan agent blocks in the OpenCode server config (lines 115-123 of index.ts). This is correct — it's the server's config format, not agent selection.
• The logOpenCodeStart function still accepts agent: 'build' | 'plan'; the hardcoded 'build' as const satisfies this type.
• Backward compatibility claim is correct: Zod's default strip mode in OpenCodeSettingsSchema.parse() silently drops the unknown agent key from existing DB records.

[nhopeatall]

Summary

LGTM — Clean, complete removal of the OpenCode agent selection feature. The hardcoded 'build' agent is correctly applied, all schema/settings/catalog/test references are consistently updated, and backward compatibility with existing DB records is preserved via Zod's strip mode.

Minor observation (not blocking): buildConfig still configures a plan agent block in the OpenCode server config, and a few internal function signatures still accept 'build' | 'plan'. These are harmless dead paths — fine to clean up in a follow-up if desired.