feat(skills): /reload-skills slash command + skills_reload agent tool

Adds a public reload path for the in-process skill caches so newly
installed (or removed) skills become visible mid-session without a
gateway restart. Mirrors the shape of /reload-mcp.

Three surfaces:
* /reload-skills slash command — CLI (cli.py) and gateway (gateway/run.py),
  with /reload_skills alias for Telegram autocomplete and an explicit
  Discord registration.
* skills_reload agent tool (tools/skills_tool.py) — lets agents/subagents
  pick up freshly-installed skills via tool call.
* agent.skill_commands.reload_skills() — shared helper that clears
  _skill_commands, _SKILLS_PROMPT_CACHE (in-process LRU), and the
  on-disk .skills_prompt_snapshot.json, then returns an added/removed
  diff plus the new total count.

Tested:
* tests/agent/test_skill_commands_reload.py (9 cases)
* tests/cli/test_cli_reload_skills.py       (3 cases)
* tests/gateway/test_reload_skills_command.py (4 cases)

Use case: NemoClaw / OpenShell-style sandboxed orchestrators that drop
skills into ~/.hermes/skills mid-session, plus agentic flows where the
agent itself installs a skill via the shell tool and needs it bound
without a gateway restart. The Python helper
clear_skills_system_prompt_cache(clear_snapshot=True) already exists
internally — this PR just exposes it via slash command and tool.

gateway/platforms/discord.py

@@ -2270,6 +2270,10 @@ class DiscordAdapter(BasePlatformAdapter):
         async def slash_reload_mcp(interaction: discord.Interaction):
             await self._run_simple_slash(interaction, "/reload-mcp")
 
+        @tree.command(name="reload-skills", description="Re-scan ~/.hermes/skills/ for new or removed skills")
+        async def slash_reload_skills(interaction: discord.Interaction):
+            await self._run_simple_slash(interaction, "/reload-skills")
+
         @tree.command(name="voice", description="Toggle voice reply mode")
         @discord.app_commands.describe(mode="Voice mode: on, off, tts, channel, leave, or status")
         @discord.app_commands.choices(mode=[