feat(plugin): single-session install + setup flow

[onlycastle]

Summary

• Add homedir() fallback for CLAUDE_PLUGIN_DATA so native deps install works without the env var
• New Step 0.5 in /bp-setup wizard: mid-session MCP server registration via claude mcp add
• Declare skills and mcpServers fields in plugin.json manifest

Changes

1. fix(plugin): fallback CLAUDE_PLUGIN_DATA to ~/.betterprompt
2. feat(plugin): add mid-session MCP registration to setup wizard
3. chore(plugin): add skills and mcpServers fields to plugin manifest
4. docs(plugin): document single-session install flow

Test Plan

• Install plugin fresh, run /bp-setup — verify Step 0.5 registers MCP server
• Confirm scan_sessions tool works after mid-session registration
• Verify existing installs (where MCP is already running) skip Step 0.5
• Check native-deps.ts fallback: unset CLAUDE_PLUGIN_DATA, verify ~/.betterprompt is used

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[onlycastle]

Code Review

Found 3 issues:

1. Dead guard after fallback addition — if (!pluginDataDir) return; on line 17 can never trigger now that the fallback || join(homedir(), '.betterprompt') guarantees a truthy value. This guard was meaningful before the fallback but is now unreachable dead code. Consider removing it or replacing with an assertion.
   

   BetterPrompt/packages/plugin/lib/native-deps.ts

   Lines 16 to 17 in cd2c011

   	const pluginDataDir = process.env.CLAUDE_PLUGIN_DATA || join(homedir(), '.betterprompt');
   	if (!pluginDataDir) return;

2. Incomplete command in PLUGIN.md diagram — The "Single-Session Install Flow" shows claude mcp add -s user betterprompt as the full command, but it's missing the required -e NODE_PATH=... and -e CLAUDE_PLUGIN_DATA=... flags plus the -- node <path> suffix. A developer reading only PLUGIN.md would get a non-working command.
   

   BetterPrompt/docs/agent/PLUGIN.md

   Line 154 in cd2c011

   install plugin → /bp-setup → Step 0.5: `claude mcp add -s user betterprompt` → MCP tools available → continue setup

3. Dual MCP registration on subsequent sessions — Step 0.5 uses claude mcp add -s user which persists across all sessions. The plugin's .mcp.json also registers the same server on subsequent sessions via the plugin lifecycle. The docs state "on subsequent sessions, the plugin's .mcp.json handles server startup normally" but don't address whether this creates a duplicate registration or how they're reconciled.
   

   BetterPrompt/docs/agent/PLUGIN.md

   Line 157 in cd2c011

   The `claude mcp add` command registers the server at user scope with `NODE_PATH` and `CLAUDE_PLUGIN_DATA` env vars pointing to `~/.betterprompt/`. On subsequent sessions, the plugin's `.mcp.json` handles server startup normally.

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