Closes #362.

Why

The official @modelcontextprotocol/server-filesystem refuses to start when its sandbox directory doesn't exist and prints a raw Node stack trace from inside npx's child:

Error accessing directory /tmp/reasonix-sandbox: ENOENT: no such file or directory, stat '/tmp/reasonix-sandbox'
    at async Object.stat (node:internal/fs/promises:1680:18)
    at async file:///home/dacec/.npm/_npx/.../server-filesystem/dist/index.js:43:23

The user has no idea what to do, and we're partly responsible: our wizard suggests /tmp/reasonix-sandbox as a placeholder, then saves whatever the user types into ~/.reasonix/config.json without checking it.

What changes

Two-layer fix.

1. Wizard input-time validation (src/cli/ui/Wizard.tsx)

When the filesystem MCP server's directory arg is submitted:

• exists & dir → accept as before
• exists but not a dir → red error, stay on input
• missing → switch to a confirm step: [Y/Enter] create it (mkdir -p) / [N/Esc] enter a different path

2. Spawn-time fallback (src/mcp/preflight.ts + 3 call sites)

preflightStdioSpec(spec) runs right before new StdioTransport(...) in chat.tsx, run.ts, and mcp-inspect.ts. If a stdio spec contains @modelcontextprotocol/server-filesystem, every positional path arg is stated; missing → throws

MCP filesystem sandbox '/tmp/reasonix-sandbox' does not exist — create it with: mkdir -p '/tmp/reasonix-sandbox'

The existing per-call-site catch already surfaces this as a failed MCP lifecycle event, so users see the actionable message in the same place they currently see the raw stack.

The spawn-time path deliberately does not auto-mkdir: at that point the user might be running with a stale config they don't remember writing, and silently creating directories on their behalf is too magic. The wizard is the right place to ask before creating.

Tests

tests/mcp-preflight.test.ts — 6 cases:

• non-filesystem servers: no-op
• existing dir: no throw
• missing dir: actionable error
• path-is-a-file: distinct error
• multi-dir spec, one missing: throws on the missing one
• flag-shaped tokens after the package id: ignored

Test plan

• npx vitest run tests/mcp-preflight.test.ts — 6/6 pass
• npx vitest run — full suite 2430 pass / 5 skipped
• tsc --noEmit clean
• biome check clean on touched files
• Manual: bunx reasonix code with a stale config pointing at a missing dir → actionable error instead of raw Node stack
• Manual: reasonix init → pick filesystem → enter a missing path → Y creates it, N returns to input