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

[cheellipadi]

Related to #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?

[natashaannn]

Discussed with cheeyang on Slack thread, need further work and new issues later on to discuss larger theme of prioritisation by PMs and their needs. To merge first and revisit discussion later.

Other questions:

• how is motivation different from background?