feat: multi-agent architecture — named agents with routing, tool policies, and isolated workspaces

Implements the full multi-agent system for Hermes Agent, allowing a single
installation to host multiple named agents, each with its own model,
personality, toolset, workspace, and session history.

## New Files

- gateway/agent_registry.py: AgentConfig, ToolPolicy, SubagentPolicy,
  AgentRegistry, TOOL_PROFILES (minimal/coding/messaging/full), and
  normalize_tool_config() for shorthand YAML parsing

- gateway/router.py: BindingRouter with 7-tier deterministic routing
  (chat_id > peer > guild+type > guild > platform+type > platform > default)

## Core Changes

- model_tools.py: get_tool_definitions() accepts agent_tool_policy for
  per-agent tool filtering; handle_function_call() extended enabled_tools
  check to gate ALL tool calls (defense-in-depth)

- gateway/session.py: build_session_key() now accepts agent_id and dm_scope
  parameters, replacing hardcoded 'agent:main' with 'agent:{agent_id}'

- tools/memory_tool.py: MemoryStore accepts memory_dir parameter for
  per-agent memory isolation

- agent/prompt_builder.py: build_context_files_prompt() accepts
  agent_workspace for SOUL.md lookup; build_skills_system_prompt()
  accepts agent_skills_dir for per-agent skill overlay

- run_agent.py: AIAgent accepts agent_tool_policy and agent_workspace,
  passes policy through to get_tool_definitions()

- gateway/run.py: Initializes AgentRegistry + BindingRouter, resolves
  agent per-message in _handle_message(), passes config to _run_agent(),
  adds /agents command

- cli.py: --agent flag for selecting named agent profiles, /agents
  slash command, agent config override for model/personality/tools

- hermes_cli/config.py: agents/bindings in DEFAULT_CONFIG, version 7

- tools/delegate_tool.py: Configurable max_depth per-agent, tool policy
  inheritance from parent to child

## Config Format

agents:
  main:
    default: true
  coder:
    model: anthropic/claude-sonnet-4
    personality: 'You are a coding assistant.'
    tools: coding  # or [tool1, tool2] or {profile: x, deny: [...]}

bindings:
  - agent: coder
    telegram: '-100123456'

## Tests

168 new tests across 3 test files (agent_registry, router, integration).
All 3106 tests pass.

hermes_cli/main.py

@@ -495,6 +495,7 @@ def cmd_chat(args):
         "resume": getattr(args, "resume", None),
         "worktree": getattr(args, "worktree", False),
         "checkpoints": getattr(args, "checkpoints", False),
+        "agent": getattr(args, "agent", None),
     }
     # Filter out None values
     kwargs = {k: v for k, v in kwargs.items() if v is not None}
@@ -1966,6 +1967,12 @@ For more help on a command:
         default=False,
         help="Bypass all dangerous command approval prompts (use at your own risk)"
     )
+    chat_parser.add_argument(
+        "--agent",
+        type=str,
+        default=None,
+        help="Named agent profile to use from config.yaml"
+    )
     chat_parser.set_defaults(func=cmd_chat)
 
     # =========================================================================