docs(memory): list all nine knowledge categories - #6864

Summary

Corrects the memory guide's category count from six to nine and adds the three missing categories: profile, system and permission.

Why

The Knowledge Categories section says there are six categories, but VALID_CATEGORIES in src/gaia/agents/base/memory_store.py has nine, and gaia memory status reports the others. The same guide already lists all nine in its Quick Start, so it contradicts itself.

I split the list because the missing three work differently: store() rejects them from a chat turn unless the caller passes allow_privileged=True.

Linked issue

Closes #3211

Changes

• Says there are nine categories, and that six can be created in chat.
• Adds a second table for profile, system and permission, and says where they come from, based on the comment above _PRIVILEGED_CATEGORIES.
• The profile example follows the template in bootstrap.py, and the permission example comes from the comment on VALID_CATEGORIES.

Test plan

• Compared the tables in Knowledge Categories with VALID_CATEGORIES: both have the same 9 names.
• Checked docs/guides/memory.mdx against open PRs fix(memory): forget every copy of a memory, and don't re-save it on a forget turn #6858 and fix(memory): teach the next chat what worked after a failed Python import #6828, which also edit it: they change other sections.
• Docs site build: not run. The change adds a standard Markdown table.

Evidence

N/A: this is a docs-only change. It doesn't touch the Agent UI, MCP tools, CLI behavior or the HTTP API.

• Agent exposed in the Agent UI: N/A
• MCP tools / servers: N/A
• CLI: N/A
• HTTP API / REST: N/A

Checklist

• I have linked a GitHub issue above (Closes #N / Fixes #N / Refs #N).
• I have described why this change is being made, not just what changed.
• I have run linting and tests locally (python util/lint.py --all, pytest tests/unit/). Not run: this PR only changes one .mdx file and no code.
• I have attached real-world evidence matched to the surface I changed (see Evidence above), or marked each surface N/A.
• I have updated documentation if user-visible behavior changed (see CONTRIBUTING.md). This PR is the documentation change.