chore: Link plan documents to created issues bidirectionally

[JeremyDev87]

Purpose

When plan-to-issues creates GitHub issues from a plan document, add bidirectional references so you can navigate from plan → issues and from issues → plan.

Changes

• `.claude/skills/plan-to-issues/SKILL.md` — Add steps to update plan document with issue numbers and add plan link to parent issue

Acceptance Criteria

• After issue creation, plan document (if file-based) is updated with a "## Created Issues" section listing all issue numbers/URLs
• Parent issue body includes a "Plan Document" link if source was a file
• Works gracefully when plan source is text (no file to update)
• Can be merged independently

Dependencies

• Follows: Recommended after plan-to-issues --jq improvement (but independently deployable)

[JeremyDev87]

Technical Details

Exact Changes

1. Update plan document after issue creation (Step 7b):

If plan source was a file:
  Append to the plan file:
  ## Created Issues
  - Parent: #NNN — <title>
  - Sub: #NNN — <title>
  - ...
  - Created: YYYY-MM-DD

2. Add plan link to parent issue:

In parent issue body, add:
## Plan Document
Source: \`docs/plans/YYYY-MM-DD-<topic>.md\`

Edge Cases

• Plan source is text (no file) → skip file update, only add note to parent issue
• Plan source is context.md → add issue numbers to context via update_context
• Plan file is outside repo → skip file update with warning

Steps

1. Add file detection logic in Step 7
2. Add plan file update step
3. Add plan link to parent issue template
4. Handle edge cases gracefully

Dependencies

• Recommended after chore: Improve plan-to-issues with --jq filtering and context.md integration #772 (plan-to-issues --jq improvement) but independently deployable