feat: PromptResolver — inline vs promptFile (#7)

[pyadav]

Closes #7.

Summary

• New resolvePrompt() in @missingstudio/sanddune-core returning a ResolvedPrompt tagged "inline" or "template" — first stage of the prompt pipeline per the project's module map.
• Inline path returns the string verbatim (no {{KEY}} scan, no shell-expression scan, no built-in argument injection — ADR-0008's "inline = literal").
• Template path reads the file from disk and returns text + the untouched promptArgs map + the resolved absolute path. Substitution and expansion are out of scope for this slice; they consume the tag in later slices.
• Runtime guards (defense in depth on the existing compile-time mutual exclusion in PromptOption):
  • prompt + promptFile → throws.
  • prompt + promptArgs → throws (per ADR-0008).
  • missing promptFile → error names the resolved absolute path.
• promptFile resolves relative paths against process.cwd() (caller's perspective), not RunOptions.cwd — per CONTEXT.md's two-perspectives rule.
• runProgram now calls the resolver in place of the previous "promptFile is not yet supported" branch and feeds resolved.text into the iteration loop. Downstream still consumes a plain string.
• README updated to list promptFile as a working option; promptArgs is silently ignored on the template path until substitution lands.

Test plan

• bun run typecheck
• bun test — 78 pass, 1 skip, 0 fail
• New unit tests cover: inline verbatim, template path, default empty promptArgs, both rejected, neither rejected, prompt + promptArgs rejected, missing file names absolute path, relative path resolves against process.cwd(), absolute path used as-is
• Existing inline integration test in run.integration.test.ts continues to pass
• Type-level negative tests in packages/sanddune/src/types.test.ts left untouched

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