Create the OpenCode config directory before writing install defaults

[Chebaleomkar]

Fixes #93

Problem

On a machine without ~/.config/opencode (plugin installed before OpenCode was ever run, fresh CI/Docker/headless boxes), install crashed with ENOENT on its first write to supermemory.json, before any of the later steps that create the folder could run. The installer did nothing and exited with a stack trace.

Fix

One line in install(): create OPENCODE_CONFIG_DIR (recursive, so a no-op when it exists) before writeInstallDefaults(). This matches what the later install steps already do (src/cli.ts:309, :323, :347). It lives in install() rather than config.ts, so it doesn't conflict with #92, which moves writeInstallDefaults into the CLI.

No behaviour change for users whose folder already exists.

Before / after

Empty HOME, install --no-tui:

# main (and published 2.0.15)
🧠 opencode-supermemory installer
Error: ENOENT: no such file or directory, open '<HOME>/.config/opencode/supermemory.json'
exit 1, ~/.config/opencode not created

# this branch
✓ Created <HOME>/.config/opencode/opencode.jsonc
✓ Enabled the persistent Supermemory footer for OpenCode V1 in <HOME>/.config/opencode/tui.jsonc
✓ Created /supermemory-index command  (… and the other 4 commands)
✓ Setup complete! Restart OpenCode (V1 and/or OpenCode 2) to activate.
exit 0

Testing

• New src/cli.test.ts runs the real install --no-tui against an empty temporary home. It fails on main with the exact ENOENT and passes with this change.
• bun run typecheck is clean and bun test passes (34 tests).
• End to end with the built CLI on an empty home, on Windows 11 and on Linux (WSL2 Ubuntu 24.04). The full install completes and creates opencode.jsonc, tui.jsonc, supermemory.json and the command files.