fix: install unresolved monorepo plugin deps

[lttlin]

Problem

Monorepo plugin installs currently run npm install --omit=dev only once at the monorepo root. When a sub-plugin declares its own runtime dependencies but the monorepo does not expose that package through root workspaces, the compiled plugin loads from ~/.opencli/monorepos/.../packages/<plugin>/... and crashes at runtime with module resolution errors such as Cannot find package 'undici'.

Root Cause

The monorepo install lifecycle assumes the root install is always sufficient for every sub-plugin. It never checks whether a sub-plugin's declared runtime dependencies are actually resolvable from that sub-plugin directory before it is finalized and linked.

Fix

Keep the existing root-level install, but add a targeted fallback for monorepo sub-plugins: if a sub-plugin's dependencies / optionalDependencies are still unresolved from its own directory after the root install, run npm install --omit=dev in that sub-plugin before finalizing it. This preserves the fast path for workspace monorepos while fixing non-workspace monorepos like the one reported in #722.

Validation

• npx vitest run src/plugin.test.ts
• Added coverage for the unresolved-runtime-dependency fallback in monorepo installs

Closes #722