feat(acp): Gemini ACP protocol standardization + provider hot-reload + Gemini embedding

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Summary

Cleanup PR for ACP (Agent Client Protocol) provider work. Addresses protocol-compatibility issues and operational improvements discovered while integrating the Gemini CLI as an ACP-compatible agent.

Changes

• ACP protocol standardization — internal/providers/acp/{process,session,tool_bridge,types}.go and tests
  • Handle session/request_permission for Gemini CLI 0.38.x
  • Prompt inactivity watchdog
  • Stealth mode option
• Unified ACPSettings config struct — both registration paths (config.json providers.acp and DB llm_providers.settings JSONB) share the same option set (name/binary/args/model/perm_mode/idle_ttl/session_ttl/prompt_timeout/work_dir/include_directories/mcp_data)
• ACP provider hot-reload — when an ACP provider row is added/updated in the DB, it is re-registered immediately without a gateway restart (SetProviderReloadFn callback + acpMCPData package var refresh)
• Provider verify refactor — POST /v1/providers/{id}/verify
  • model is now required (ping mode removed)
  • Response shape: {success, models[]}
  • CLI/onboard: --model flag removed; verification prints the list of available models
• Gemini native embedding provider — adds memory.NewGeminiEmbeddingProvider and a ProviderGeminiNative branch in buildEmbeddingProvider

Out of scope (split into follow-up PRs)

• MCP bridge BuiltinToolStore-based tool selection refactor (internal/mcp/bridge_server.go, internal/gateway/server.go BuiltinTools wire)
• Korean i18n (locale=ko)
• Channel watchdog / bot-token masking
• Consolidation episodic timeout, shellDenyGroups
• Heartbeat FK rollback (migration 57)

Test plan

• go build ./... (PG) passes
• go build -tags sqliteonly ./... (Desktop/SQLite) passes
• go vet ./... passes
• Integration tests (go test -race -tags integration ./tests/integration/) — to be verified in reviewer environment
• Manually verify Gemini CLI (0.38.x) ACP session-create / request-permission flow
• Verify ACP provider hot-reload: add an ACP provider via DB and confirm it works without gateway restart
• Verify goclaw providers verify <id> prints the available model list

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Superseded by #1069 (full integrated branch with ko i18n + ops hardening + ACP). Closing this narrower PR to consolidate review on a single branch.