feat: add explicit local PRD/task ingestion flags

[rogerchappel]

Summary

• add explicit --prd <path> and --tasks <path> flags to stackforge init
• copy local planning docs into generated repos as docs/PRD.md and docs/TASKS.md
• add smoke coverage for dry-run planning and copied doc outputs

Verification

• Tests or checks run:
  • pnpm check
  • pnpm build
  • pnpm check:templates
  • pnpm smoke:init
• Manual review completed:
  • reviewed copied PRD/tasks handling, dry-run behavior, and missing-file error path

Risk Level

• Low
• Medium
• High

Notes:

• local PRD/tasks inputs are copied verbatim and do not go through template variable rendering
• no network calls were added for this slice
• rebased onto latest origin/main before updating this PR

Rollback Plan

• revert commit 1b2411c to remove the new init flags and smoke coverage

Human Decision Needed

• None
• Maintainer review
• Product/design decision
• Security/privacy review
• Other: