chore: remove unused imports, dead code, and stale comments

Mechanical cleanup — no behavior changes.

Unused imports removed:
- model_tools.py: import os
- run_agent.py: OPENROUTER_MODELS_URL, get_model_context_length
- cli.py: Table, VERSION, RELEASE_DATE, resolve_toolset, get_skill_commands
- terminal_tool.py: signal, uuid, tempfile, set_interrupt_event,
  DANGEROUS_PATTERNS, _load_permanent_allowlist, _detect_dangerous_command

Dead code removed:
- toolsets.py: print_toolset_tree() (zero callers)
- browser_tool.py: _get_session_name() (never called)

Stale comments removed:
- toolsets.py: duplicated/garbled comment line
- web_tools.py: 3 aspirational TODO comments from early development

run_agent.py

@@ -70,7 +70,7 @@ from tools.browser_tool import cleanup_browser
 
 import requests
 
-from hermes_constants import OPENROUTER_BASE_URL, OPENROUTER_MODELS_URL
+from hermes_constants import OPENROUTER_BASE_URL
 
 # Agent internals extracted to agent/ package for modularity
 from agent.prompt_builder import (
@@ -78,7 +78,7 @@ from agent.prompt_builder import (
     MEMORY_GUIDANCE, SESSION_SEARCH_GUIDANCE, SKILLS_GUIDANCE,
 )
 from agent.model_metadata import (
-    fetch_model_metadata, get_model_context_length,
+    fetch_model_metadata,
     estimate_tokens_rough, estimate_messages_tokens_rough,
     get_next_probe_tier, parse_context_limit_from_error,
     save_context_length,