refactor: remove postConfigure hook and make sequential gadget execution unconditional

[aaight]

Summary

• Remove the postConfigure hook mechanism entirely (YAML field, schema field, registry, resolution logic, builder factory option, and frontend UI dropdown)
• Make sequential gadget execution unconditional: withGadgetExecutionMode('sequential') is now always called in createConfiguredBuilder() for all agents
• Update all related tests to reflect the new behavior

Changes

• src/agents/shared/builderFactory.ts: Remove postConfigure from CreateBuilderOptions interface, remove conditional hook call, add unconditional .withGadgetExecutionMode('sequential')
• src/backends/llmist/index.ts: Remove POST_CONFIGURE_REGISTRY constant, resolveAgentDefinition import, and postConfigure IIFE from builder options
• src/agents/definitions/schema.ts: Remove postConfigure field from BackendSchema
• src/agents/definitions/implementation.yaml: Remove postConfigure: sequentialGadgetExecution
• web/src/components/settings/agent-definition-editor.tsx: Remove Post-Configure Hook dropdown and unused Select imports
• tests/unit/agents/definitions/schema.test.ts: Remove two postConfigure-specific tests, update optional fields test
• tests/unit/agents/definitions/loader.test.ts: Remove postConfigure spot-check tests
• tests/unit/agents/shared/builderFactory.test.ts: Add withGadgetExecutionMode mock, replace postConfigure tests with sequential execution test

Test plan

• All unit tests pass (463 tests)
• TypeScript type check passes
• Biome lint passes with no issues

Card: https://trello.com/c/69a5aed609d2220335221dcb

Generated with Claude Code

[nhopeatall]

✨ On it — checking the PR changes now

[nhopeatall]

LGTM — Clean removal of a single-use abstraction. The postConfigure hook had exactly one implementation (sequentialGadgetExecution) used by one agent. Making sequential execution unconditional in createConfiguredBuilder() eliminates the registry, YAML field, schema field, IIFE resolution in the llmist backend, and the frontend dropdown — all without losing functionality. The behavioral change (all agents now get sequential gadget execution, not just implementation) is intentional per the PR description and all CI checks pass.