Add CLAUDE.md template with session-start /index . instruction

## Summary

Document the recommended `CLAUDE.md` template that consumers of the `semantic-memory` plugin should place at the root of their workspaces. The template enforces an incremental `/index .` at session start and explains the available slash commands and namespaces.

This issue tracks shipping the snippet below as a copy-paste-ready section in the plugin docs (and optionally in `skills/semantic-memory/SKILL.md` / a `templates/CLAUDE.md` reference file) so users get a consistent first-run experience.

## Proposed `CLAUDE.md` snippet

```markdown
# CLAUDE.md

## Semantic Memory (LogosDB)

The **semantic-memory** plugin is active for this workspace. Vector data lives in `.logosdb/` (git-ignored). The plugin provides three slash commands that work without any extra setup.

### Session start — mandatory

**Run `/index .` at the start of every session** before doing other work. This incrementally refreshes the full index (only changed/new files are re-embedded; unchanged files are skipped, so it is fast).

```
/index .
```

### Slash commands

| Command | What it does |
|---------|--------------|
| `/index ` | Index or re-index a file or directory (incremental by default). Use `.` for the whole project, or a subdirectory/file for targeted refresh. |
| `/search ` | Semantic search over indexed content. Returns ranked file matches. Accepts `--top-k=n` (default 5), `--namespace=name` (default `code`), and optional ISO timestamp bounds (`--from-ts`, `--to-ts`). |
| `/forget ` | Delete indexed chunks by semantic query match or by row id. Use to remove stale or unwanted content from the index. |

### Namespaces

- **`code`** (default) — source files and general project content
- Use `--namespace=docs` or `-n docs` for documentation-only searches
- Use `--namespace=decisions` for durable architectural or research notes

### When to re-index

- After pulling / merging changes: `/index .`
- After editing a specific file: `/index `
- Before a broad search when files may have changed since last index

### Opting out of auto-index

If the repo grows very large and `.` is too slow, replace `/index .` in the session-start instruction above with a narrower path (e.g. `/index ./src`) and document that here.
```

## Acceptance criteria

- [ ] `README.md` references the snippet (or links to a `templates/CLAUDE.md`) under the existing `Slash commands` section.
- [ ] `skills/semantic-memory/SKILL.md` mentions the session-start `/index .` contract and points to the template.
- [ ] Verify `/index .` with `incremental: true` is the documented default and matches `logosdb-mcp-server >= 0.7.11` behavior.
- [ ] Note in docs that `.logosdb/` should be git-ignored.

## Notes

- Keeps the contract simple: one mandatory command at session start, three slash commands, three namespaces.
- The opt-out section gives an escape hatch for very large repos without changing plugin defaults.