feat: candidate differentiation from other likely applicants

[warnes]

Sub-issue of #80 (item 4 — "Differentiation from other likely candidates").

Context

Partially addressed today by the persuasion-expert review: tasks/user-story-persuasion-expert.md defines a "differentiation from generics" criterion (no generic-summary phrasing; a rare skill combination should contrast the candidate with the typical applicant pool), and GAP-104 (closed) added pattern-interrupt cover-letter openers.

This item proposes a broader, explicit differentiation capability for the application package.

Proposed direction (broad; refine before building)

• Generate explicit "why this candidate" differentiators vs the likely applicant pool for a given role.
• Suggest supporting evidence (rare skill combinations, domain-specific wins) to weave into summary, bullets, and cover letter.
• Keep it a user-reviewable suggestion layer (no silent edits to generated files).

Acceptance criteria

• Differentiator assessment surfaced as structured, user-reviewable suggestions.
• Reuse/close the loop with existing persuasion-expert differentiation criteria rather than duplicating them.
• No silent writes to Master CV or generated documents (consistent with Add optional in-app persona review for generated documents #60's safety rule).

Related

• Parent: Feature: Potential Additional Workflow Steps #80
• Existing: tasks/user-story-persuasion-expert.md (differentiation signal)
• Closed precursor: GAP-30: Cover letter opening hardwired as "Dear [name]," — blocks pattern-interrupt openers #104 (pattern-interrupt openers)