feat: add configuration system with JSONC support

[kldzj]

Summary

• Add a 2-layer configuration system (global ~/.config/opencode/btw.jsonc + project .opencode/btw.jsonc) allowing users to customize hint behavior without code changes
• Include JSON Schema (btw.schema.json) for IDE auto-completion out of the box
• Auto-creates global config with $schema reference on first run

Configuration Options

Option	Default	Description
defaultPinned	false	Default hint type (transient vs pinned)
autoClear.onIdle	true	Clear transient hints on session idle
autoClear.onQuestionTool	true	Clear transient hints on question tool
injection.target	"both"	Where hints are injected: "both", "system", "user"
injection.systemPromptPosition	"prepend"	System prompt placement: "prepend" or "append"
injection.systemInstructions	(built-in)	Custom system prompt framing text
injection.userMessagePrefix	"BTW, "	Custom user message prefix
debug	false	Default debug mode
toastDuration	3000	Default toast duration in ms

Changes

• New: src/config.ts — config types, defaults, 2-layer JSONC loading, validation with advisory warnings
• New: src/config.test.ts — comprehensive config tests
• New: btw.schema.json — JSON Schema for IDE support
• Modified: src/core.ts — functions accept optional config parameter
• Modified: src/plugin.ts — loads config at init, all hooks respect config
• Modified: src/core.test.ts — updated imports + config-aware tests
• Modified: package.json — added jsonc-parser dependency
• Modified: README.md — added Configuration section with examples
• Modified: .github/workflows/test.yml — updated branch refs from master to main

Test Coverage

177 tests, 293 assertions across 3 test files — all passing.