docs: cross-link write-approval gate from skills, configuration, and slash-command docs (#43801)

The memory/skill write-approval gate (#38199, #43354, #43452) was only
documented inside features/memory.md. Surface it everywhere users will
actually look:

- features/skills.md: new 'Gating agent skill writes' section under
  skill_manage, with the staging semantics, review commands, and the
  distinction from skills.guard_agent_created
- configuration.md: memory.write_approval added to the Memory
  Configuration block; new 'Write approval for skill writes' subsection
  next to the guard_agent_created scanner
- reference/slash-commands.md: /memory and /skills review subcommands in
  both the CLI and messaging tables; Notes updated since /skills
  pending/approve/reject/diff/approval now works on the gateway
- features/memory.md: cross-link to the new skills section

website/docs/user-guide/features/memory.md

@@ -270,6 +270,7 @@ inline, but the full diff stays out-of-band:
 On a messaging platform, approve a skill from its gist + metadata, or open
 `/skills diff` on the CLI / dashboard / the staged file under
 `~/.hermes/pending/skills/<id>.json` when you want to read the whole change.
+Full details in [Gating agent skill writes](/user-guide/features/skills#gating-agent-skill-writes-skillswrite_approval).
 
 
 ## External Memory Providers