fix: use openclaw plugins install for plugin setup (#167)

* fix: use openclaw plugins install instead of npm install -g for plugin setup

The SKILL.md install instructions used npm install -g + manual load.paths
config, causing the plugin to register as origin:"config" instead of
origin:"installed". This made openclaw doctor unable to detect palaia as
the active memory plugin.

Switched all install/update paths to openclaw plugins install:
- SKILL.md frontmatter (postInstall, postUpdate)
- Manual plugin setup instructions
- Manual update step 4
- palaia upgrade CLI command (cli.py)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: add OpenClaw plugin install to manual setup in README

The manual setup section only had pip install + palaia init but was
missing the openclaw plugins install step and memory slot activation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>