feat(aif-create-issue): add Motivation section to issue template

[cheellipadi]

Closes #81

Summary

• Adds a `## Motivation` section as the first author section in `issue-template.md`
• Updates SKILL.md Step 1 to prompt for Motivation (item 2, before user story)
• Updates the author sections summary line to include motivation

Changes

• `skills/aif-create-issue/issue-template.md`: new `## Motivation` section added before `## User story`
• `skills/aif-create-issue/SKILL.md`: Motivation added as gather step 2; list renumbered 1-7; author sections description updated

Test plan

• Scenario 1 (Motivation section appears): issue-template.md now contains `## Motivation` as the first author section -- any issue created from the template will include it
• Scenario 2 (Developer is prompted): SKILL.md Step 1 now lists Motivation as a required gather item with the same "do not invent, ask" instruction as other required sections

Test Evidence

#83 created using aif-create-issue with changeset from this PR

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🤖 Generated with aif-implement-issue

[natashaannn]

To think about this in the larger scheme of issue prioritization - what can we write in 'Motivation' section that helps PMs and engineers quantitatively account for priority of feature?