feat(plugin): coding-focused custom-category setup script

mem0 auto-tags every memory with one or more `categories` from a
project-level list. The default list is consumer-oriented (food,
hobbies, music, ...), which produces meaningless tags for code
workflows.

Per-request `custom_categories` is not supported on the managed API,
so the fix has to be a one-time project-level configuration. New
script `setup_coding_categories.py` does this:

  - Dry-run by default: prints current vs proposed taxonomy, exits.
  - `--apply` flag actually calls `project.update(custom_categories=...)`.
  - Falls back gracefully when mem0ai SDK isn't installed or
    MEM0_API_KEY is missing/invalid (friendly error, no stack trace).

Recommended taxonomy:
  architecture_decisions, anti_patterns, task_learnings,
  tooling_setup, bug_fixes, coding_conventions, user_preferences

Skill clarification: `metadata.type` (agent-applied explicit tag,
used in filters) and `categories` (platform-applied auto-tag, used
in dashboards) are complementary; agent should keep using
`metadata.type` and not try to set `categories` per-request.

README: section under Step 2 explaining how to run the script.

mem0-plugin/skills/mem0-mcp/SKILL.md

@@ -97,6 +97,8 @@ Extract key learnings and store them using the `add_memory` tool:
 - **Environment/setup discoveries** -> Include metadata `{"type": "environmental"}`
 - **Conventions established** -> Include metadata `{"type": "convention"}`
 
+> `metadata.type` (which you set explicitly) and `categories` (which the platform auto-tags after the project's custom-category list — see `scripts/setup_coding_categories.py`) are complementary. Always set `metadata.type` for explicit filtering; the platform fills in `categories` on its own. Don't try to set `categories` on `add_memory` calls — per-request overrides aren't supported on the managed API.
+
 Memories can be as detailed as needed -- include full context, reasoning, code snippets, file paths, and examples. Longer, searchable memories are more valuable than vague one-liners.
 
 ### Use `infer=False` for already-structured content