Add Plugin API v3.6.0 config reload support

[dmortondev]

Summary

Implements the Config Reload request/response pair from Plugin API v3.6.0 (Launcher v2.19.0), allowing the Launcher to request plugins to reload their configuration at runtime without restarting.

• Add ConfigReloadablePlugin optional interface — plugins implement ReloadConfig(ctx context.Context) error to handle reload requests
• Add ConfigReloadError type for classified reload failures (Load, Validate, Save) and ConfigReloadErrorType enum
• Plugins that don't implement ConfigReloadablePlugin automatically send a success response
• Wire protocol: message type 202, response includes errorType (int enum) and errorMessage (string)

Changes

Area	Files
API types	api/types.go, api/types_test.go
Protocol	internal/protocol/rpc.go, internal/protocol/stream.go, internal/protocol/rpc_test.go
Plugin interface & handler	launcher/launcher.go, launcher/launcher_test.go
Test utilities	plugintest/mocks.go
Docs	AGENTS.md, README.md, CHANGELOG.md, docs/API.md, docs/ARCHITECTURE.md, docs/GUIDE.md

Design decisions

• Optional interface pattern: Follows existing BootstrappedPlugin, MultiClusterPlugin, and LoadBalancedPlugin patterns — type assertion in the handler dispatch
• Error classification: ReloadConfig returns error; plugins return *ConfigReloadError for classified errors (Load/Validate/Save) or plain error for unclassified failures (mapped to ReloadErrorUnknown)
• RequestFromJSON export: Renamed from requestFromJSON to enable handler dispatch tests without depending on internal struct construction

Test plan

• just build — compiles cleanly
• just test — all tests pass (16 new tests)
• just lint — 0 issues
• just build-examples — examples still build
• Protocol round-trip: ConfigReloadRequest deserializes from JSON, ConfigReloadResponse serializes with correct messageType, errorType, errorMessage
• Handler dispatch: not-implemented (auto-success), success, ConfigReloadError, plain error, wrapped ConfigReloadError
• ConfigReloadErrorType.String() covers all enum values plus invalid input