feat: configurable subagent provider:model with full credential resolution

Adds delegation.model and delegation.provider config fields so subagents
can run on a completely different provider:model pair than the parent agent.

When delegation.provider is set, the system resolves the full credential
bundle (base_url, api_key, api_mode) via resolve_runtime_provider() —
the same path used by CLI/gateway startup. This means all configured
providers work out of the box: openrouter, nous, zai, kimi-coding,
minimax, minimax-cn.

Key design decisions:
- Provider resolution uses hermes_cli.runtime_provider (single source of
  truth for credential resolution across CLI, gateway, cron, and now
  delegation)
- When only delegation.model is set (no provider), the model name changes
  but parent credentials are inherited (for switching models within the
  same provider like OpenRouter)
- When delegation.provider is set, full credentials are resolved
  independently — enabling cross-provider delegation (e.g. parent on
  Nous Portal, subagents on OpenRouter)
- Clear error messages if provider resolution fails (missing API key,
  unknown provider name)
- _load_config() now falls back to hermes_cli.config.load_config() for
  gateway/cron contexts where CLI_CONFIG is unavailable

Based on PR #791 by 0xbyt4 (closes #609), reworked to use proper
provider credential resolution instead of passing provider as metadata.

Co-authored-by: 0xbyt4 <0xbyt4@users.noreply.github.com>

cli-config.yaml.example

@@ -626,6 +626,10 @@ code_execution:
 delegation:
   max_iterations: 50                          # Max tool-calling turns per child (default: 50)
   default_toolsets: ["terminal", "file", "web"]  # Default toolsets for subagents
+  # model: "google/gemini-3-flash-preview"    # Override model for subagents (empty = inherit parent)
+  # provider: "openrouter"                    # Override provider for subagents (empty = inherit parent)
+  #                                           # Resolves full credentials (base_url, api_key) automatically.
+  #                                           # Supported: openrouter, nous, zai, kimi-coding, minimax
 
 # =============================================================================
 # Honcho Integration (Cross-Session User Modeling)