feat(skills-guard): gate agent-created scanner on config.skills.guard_agent_created (default off)

Replaces the blanket 'always allow' change from the previous commit with
an opt-in config flag so users who want belt-and-suspenders security can
still get the keyword scan on skill_manage output.

## Default behavior (flag off)
skill_manage(action='create'|'edit'|'patch') no longer runs the keyword
scanner. The agent can write skills that mention risky keywords in prose
(documenting what reviewers should watch for, describing cache-bust
semantics in a PR-review skill, referencing AGENTS.md, etc.) without
getting blocked.

Rationale: the agent can already execute the same code paths via
terminal() with no gate, so the scan adds friction without meaningful
security against a compromised or malicious agent.

## Opt-in behavior (flag on)
Set skills.guard_agent_created: true in config.yaml to get the original
behavior back. Scanner runs on every skill_manage write; dangerous
verdicts surface as a tool error the agent can react to (retry without
the flagged content).

## External hub installs unaffected
trusted/community sources (hermes skills install) always get scanned
regardless of this flag. The gate is specifically for skill_manage,
which only agents call.

## Changes
- hermes_cli/config.py: add skills.guard_agent_created: False to DEFAULT_CONFIG
- tools/skill_manager_tool.py: _guard_agent_created_enabled() reads the flag;
  _security_scan_skill() short-circuits to None when the flag is off
- tools/skills_guard.py: restore INSTALL_POLICY['agent-created'] =
  ('allow', 'allow', 'ask') so the scan remains strict when it does run
- tests/tools/test_skills_guard.py: restore original ask/force tests
- tests/tools/test_skill_manager_tool.py: new TestSecurityScanGate class
  covering both flag states + config error handling

## Validation
- tests/tools/test_skills_guard.py + test_skill_manager_tool.py: 115/115 pass
- E2E: flagged-keyword skill creates with default config, blocks with flag on

hermes_cli/config.py

@@ -760,6 +760,17 @@ DEFAULT_CONFIG = {
         "inline_shell": False,
         # Timeout (seconds) for each !`cmd` snippet when inline_shell is on.
         "inline_shell_timeout": 10,
+        # Run the keyword/pattern security scanner on skills the agent
+        # writes via skill_manage (create/edit/patch).  Off by default
+        # because the agent can already execute the same code paths via
+        # terminal() with no gate, so the scan adds friction (blocks
+        # skills that mention risky keywords in prose) without meaningful
+        # security.  Turn on if you want the belt-and-suspenders — a
+        # dangerous verdict will then surface as a tool error to the
+        # agent, which can retry with the flagged content removed.
+        # External hub installs (trusted/community sources) are always
+        # scanned regardless of this setting.
+        "guard_agent_created": False,
     },
 
     # Honcho AI-native memory -- reads ~/.honcho/config.json as single source of truth.