feat(agents): add Mistral Vibe agent provider (F109)

[pocky]

Summary

• Add mistral_vibe as a new agent provider, integrating Mistral's vibe CLI into the AWF agent execution pipeline alongside the six existing providers
• Implement MCP proxy support for Mistral Vibe via a temporary VIBE_HOME/config.toml, including user config sanitization to strip pre-existing [[mcp_servers]] entries and copy credentials before injection
• Extend the base CLI executor with RunWithEnv to support per-provider environment variable overrides (required for VIBE_HOME isolation), and wire it through baseCLIProvider
• Remove stale ZPM knowledge base artifacts from prior feature branches and rename CLAUDE.md to AGENTS.md for consistency with the agent-centric documentation model

Changes

Agent Provider — Mistral Vibe

• internal/infrastructure/agents/mistral_vibe_provider.go: New MistralVibeProvider implementing ports.AgentProvider; includes Execute, ExecuteConversation, MCP injector, TOML config writer, user config sanitizer, and option validation
• internal/infrastructure/agents/options.go: Add Mistral Vibe provider option types and constructor helpers
• internal/infrastructure/agents/registry.go: Register mistral_vibe provider in the central provider registry

CLI Executor — Environment Support

• internal/infrastructure/agents/cli_executor.go: Add RunWithEnv method to ExecCLIExecutor for injecting additional environment variables into subprocess commands
• internal/infrastructure/agents/base_cli_provider.go: Add cliEnvExecutor interface, cliProviderEnv helper, and runCLIWithProviderEnv dispatcher; wire env support into both execute and executeConversation paths

Tests

• internal/infrastructure/agents/mistral_vibe_provider_unit_test.go: Unit tests covering argument building, MCP injection, config sanitization, option validation, and output extraction
• internal/infrastructure/agents/provider_options_test.go: Extended to cover Mistral Vibe option constructors
• internal/infrastructure/agents/registry_test.go: Updated to assert mistral_vibe is present in the registry
• tests/integration/cli/mistral_vibe_functional_test.go: Functional integration tests for the provider end-to-end
• tests/integration/cli/run_agent_test.go: Extended run-agent integration tests to cover the new provider

Documentation

• docs/user-guide/agent-steps.md: Add mistral_vibe to provider reference tables, token-logging table, and troubleshooting section
• docs/user-guide/workflow-syntax.md: Add mistral_vibe and github_copilot to provider option table and conversation-mode notes
• docs/user-guide/mcp-proxy.md: Document Mistral Vibe MCP integration mechanism and tool-name prefixing behavior (awf-proxy_<tool>)
• docs/development/creating-agent-provider.md: Update provider creation guide to reflect new patterns
• README.md: Update provider list
• docs/README.md: Update provider count and references

Housekeeping

• .zpm/kb/pr_feature_f102_*/, .zpm/kb/pr_feature_f103_*/, .zpm/kb/pr_feature_f105_*/, .zpm/kb/pr_feature_f106_*/, .zpm/kb/pr_feature_f107_*/: Remove stale ZPM knowledge base artifacts from merged feature branches
• .zpm/mounts.json: Update mounts configuration
• CLAUDE.md → AGENTS.md: Rename project instruction file
• .gitignore: Add .codex/ to the AI assistant local config ignore list

Test plan

• Run unit tests: go test ./internal/infrastructure/agents/...
• Run integration tests (requires vibe binary in PATH): go test -tags=integration ./tests/integration/cli/ -run TestMistralVibe
• Validate a workflow YAML using mistral_vibe as provider: awf validate
• Confirm awf run correctly dispatches to the vibe binary with expected arguments

Closes #375

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