Support package alias references in all frontmatter file fields

[sbaker]

Problem

Only inherits: resolves @alias/path package references via the using: block. All other frontmatter fields that accept file paths treat the value as a plain relative path, causing compilation failures.

Example that fails:

using:
  - name: "@prompd/base-llm-evaluator@0.1.0"
    prefix: "@eval"
inherits: "@eval/prompts/base-evaluator.prmd"   # works
system:
  - "@eval/systems/evaluator.md"                 # fails - treated as relative path
  - "../systems/safety-evaluator.md"             # works - plain relative path

Error:

Failed to extract system file @prompd/base-llm-evaluator@0.1.0/systems/evaluator.md:
Context file not found: .../prompts/@prompd/base-llm-evaluator@0.1.0/systems/evaluator.md

The alias rewriting stage correctly expands @eval/ to @prompd/base-llm-evaluator@0.1.0/, but the system file resolver then treats the expanded value as a relative filesystem path instead of a package reference.

Expected Behavior

All frontmatter fields that reference files should resolve @alias/path package references the same way inherits: does:

• system:
• user:
• context: / contexts:
• files:
• override: values (e.g., override: { system: "@eval/systems/custom.md" })

Impact

Without this, child prompts that inherit from a package cannot reference the parent package's auxiliary files (system prompts, context files) using the alias syntax. Users must either inline the content or use fragile relative filesystem paths that break when distributed as packages.