Add provider plugin architecture

[hidetzu]

Summary

• Introduce plugin-based provider extensibility for multi-VCS support
• GitHub remains built-in; external providers are discovered as prism-provider-<name> binaries on PATH
• Add --provider flag for explicit provider selection (auto-detected from URL if omitted)
• Define provider plugin protocol v1 with versioned JSON schema
• Version bump to 0.2.0-alpha.1

Changes

New files

• internal/provider/plugin/plugin.go — Plugin executor (subprocess + JSON parse + fileutil enrichment)
• internal/provider/plugin/plugin_test.go — TestHelperProcess pattern tests
• internal/provider/registry.go — Provider registry (built-in GitHub + PATH plugin discovery)
• internal/provider/registry_test.go — Auto-detection and resolution tests
• docs/provider-plugin-protocol.md — Plugin protocol specification (the contract)
• docs/adr/0001-provider-plugin-architecture.md — Architecture decision record
• docs/implementation/v0.2.0-provider-plugin.md — Implementation guide

Modified files

• cmd/prism/main.go — Replace newProvider() with registry-based resolveProvider(), add --provider persistent flag
• cmd/prism/integration_test.go — Update error assertions for auto-detection, add unknown provider test
• cmd/prism/main_test.go — Add --provider flag test
• internal/provider/github/github.go — Remove compile-time interface check (fixes import cycle)
• README.md — Add Providers section with --provider usage and plugin docs
• docs/spec.md — Add Global Flags section, fix PRISM_CONFIG env var name
• docs/architecture.md — Add Registry/Plugin Executor to diagram and description

Design decisions (ADR-0001)

• GitHub built-in — go install works out of the box
• External providers as plugins — No AWS SDK or other heavy deps in prism core
• Future: GitHub can also be extracted to plugin if needed

Test plan

• All existing tests pass (no regressions)
• Plugin executor: valid JSON, invalid JSON, wrong version, timeout, binary not found
• Registry: GitHub auto-detect, unknown host error, missing plugin error
• CLI: --provider flag exists, unknown provider error message
• make test (race), make lint, make vet all pass