fix(schema): reject interpolated cwd placeholders

[blain3white]

Summary

Reject interpolation placeholders in phase.cwd during schema validation.

cwd is documented as a literal path (or a reserved workspace keyword like temp, dedicated, worktree), but flows could still pass values like {args.repo_dir}. At runtime this eventually surfaced as a misleading spawn ... node ENOENT, which looked like a missing Node binary even though the real problem was an invalid working directory.

This change fails fast with a clear validation error instead.

Changes

• add a dedicated validation check for interpolation placeholders in phase.cwd
• keep literal paths and workspace keywords valid
• add regression coverage in schema.test.ts

Example

Before:

{
  "name": "x",
  "phases": [
    { "id": "p", "type": "agent", "task": "t", "cwd": "{args.repo_dir}" }
  ]
}

Could pass validation and later fail with:

spawn /.../node ENOENT

After:

Phase 'p': 'cwd' does not support interpolation placeholders ({args.repo_dir}).
Use a literal path, or a reserved workspace keyword ('temp', 'dedicated', 'worktree').

Testing

• PI_TASKFLOW_BUILTIN_AGENTS_DIR= node --conditions=development --experimental-strip-types --test packages/taskflow-core/test/schema.test.ts
• pnpm --filter taskflow-core build
• pnpm --filter pi-taskflow build