feat(agents): add GitHub Copilot agent provider

[pocky]

Summary

• Add github_copilot as the 6th agent provider, implementing the full AgentProvider interface via the copilot CLI binary with single-turn and multi-turn (session resume) support
• Support provider-specific options including model, mode (interactive/plan/autopilot), effort (low/medium/high), allowed_tools, denied_tools, and allow_all, with enum validation at awf validate time
• Integrate GitHub Copilot into the unified display-event abstraction, parsing JSONL streaming output (assistant.message_delta, tool.execution_start) through parseCopilotDisplayEvents
• Update documentation, CHANGELOG, README, and user guides to reflect 6-provider support and Copilot-specific authentication and configuration options

Changes

Provider Implementation

• internal/infrastructure/agents/copilot_provider.go: New CopilotProvider struct implementing AgentProvider via baseCLIProvider hooks; builds CLI args for single-turn (-p <prompt> --output-format=json --silent) and multi-turn (--resume=<id>) modes; inlines system prompt for first turn (no --system-prompt flag in Copilot CLI); validates mode and effort enums; parses JSONL display events and extracts assistant.message content; graceful fallback to raw output when session ID extraction fails
• internal/infrastructure/agents/options.go: Add CopilotProviderOption functional option type and WithCopilotExecutor option for test injection
• internal/infrastructure/agents/registry.go: Register CopilotProvider in RegisterDefaults() as the 6th provider
• internal/infrastructure/agents/stream_filter.go: Minor update to accommodate new provider integration
• pkg/display/event.go: Update display event handling to support Copilot JSONL event types

Tests

• internal/infrastructure/agents/copilot_provider_unit_test.go: Unit tests for CLI arg construction, name/validate, and option delegation
• internal/infrastructure/agents/copilot_provider_delegation_test.go: Delegation tests verifying Execute and ExecuteConversation correctly delegate to baseCLIProvider through hooks, covering first-turn, resume-turn, and system-prompt-inlining scenarios
• internal/infrastructure/agents/copilot_provider_extract_test.go: Tests for extractCopilotTextContent and extractCopilotSessionID covering valid JSONL, missing fields, empty output, and camelCase sessionId key
• internal/infrastructure/agents/copilot_provider_parse_display_events_test.go: Tests for parseCopilotDisplayEvents covering assistant.message_delta, tool.execution_start, unknown event types, and missing optional fields
• internal/infrastructure/agents/copilot_provider_validation_unit_test.go: Enum validation tests for mode and effort options
• internal/infrastructure/agents/provider_options_test.go: Updated to include CopilotProviderOption coverage
• internal/infrastructure/agents/registry_test.go: Updated registry assertions to expect 6 providers

Documentation

• CHANGELOG.md: Add F089 entry documenting all Copilot provider capabilities
• CLAUDE.md: Add architecture rule for provider name prefixes on helper methods; update provider count to 6; remove stale rule
• README.md: Add GitHub Copilot to feature bullets and provider lists
• docs/README.md: Update agent-steps link description to include GitHub Copilot
• docs/user-guide/agent-steps.md: Add GitHub Copilot section with YAML example, all supported options, authentication methods; update provider count to 6 throughout
• docs/user-guide/conversation-steps.md: Add github_copilot to provider table and session resume flag documentation

Test plan

• make test-unit passes with all new Copilot provider tests green
• awf validate reports enum errors for invalid mode/effort values on a Copilot agent step
• With copilot CLI installed and authenticated, awf run executes a single-turn github_copilot step and returns agent output
• Multi-turn conversation with continue_from correctly passes --resume=<session-id> on subsequent turns

Closes #328

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