gemma-mnemara 0.12.3 — README sync (/context pin-management suite docs)

What changed

Docs-only patch. Forward-ports the /context viewer documentation from the Claude-backed mainline so the gemma README accurately describes the pin-management suite that's been in the code since 0.12.0:

• Position-prefix badges (#03) on pinned slots
• Read-only-header / editable-body split in the detail panel
• [✏️ Rename] — rename a pinned slot's slug while preserving its position prefix
• [➕ Add slot] — create a new pinned slot at a chosen position
• [⇅ Move] — reorder pinned slots

Closes the docs gap between github and PyPI. No source changes.

pip install --upgrade gemma-mnemara