Install: one-step setup for FlyBot plugins on DSH #7

Part of #3 (umbrella).

Problem

Installing FlyBot on a real DSH web profile is not smooth yet:

1. ds-bot, @flybot/social and @flybot/tree are separate packages and each must be added:

   npx @deepseek-ai/dsh@0.2.0-rc.2 plugin --profile web add "github:Birfy/FlyBot#flybot-plugins&path:/ds-bot"
   npx @deepseek-ai/dsh@0.2.0-rc.2 plugin --profile web add "github:Birfy/FlyBot#flybot-plugins&path:/ds-bot/plugins/social"
   npx @deepseek-ai/dsh@0.2.0-rc.2 plugin --profile web add "github:Birfy/FlyBot#flybot-plugins&path:/ds-bot/plugins/tree"

2. Installed plugins are not enabled automatically; social and tree had to be added to ~/.dsh/profiles/web/cordis.patch.yml, including the Bot Social server URL.
3. On first real start ds-bot failed with cannot set property "flybot" without provide (fixed: ctx.provide).
4. A "plugin load failed" error was then reported in the web UI with other, older plugins present (e.g. dsh-plugin-agentdescent@0.1.0, incompatible with this DSH version). Being investigated with a clean profile.

Proposed

• Single install that pulls in social and tree (or a flybot meta-package) and enables them.
• Ship default config (server URL) and a first-run setup screen instead of editing cordis.patch.yml.
• Clear error in the UI when one plugin fails, without breaking the others.
• Document a clean install in README, including Node >= 22.19 and pnpm.
• Smoke test in CI that loads all three plugins on real DSH/Cordis.