feat: add interview-coach skill for PM job search coaching

[dbhat93]

New Skill: interview-coach (pm-toolkit)

An adaptive PM interview coaching system that covers the full job search lifecycle — behavioral and case mock interviews, JD decoding, storybank management, transcript analysis, and offer negotiation.

Unlike generic question banks, it scores answers across five dimensions (Substance, Structure, Relevance, Credibility, Differentiation), diagnoses root causes behind weak spots, and routes to targeted drills based on the candidate's specific gaps.

Commands included

Command	What it does
kickoff	Onboard, build profile, output prioritized action plan
prep [company]	Company research + role-specific prep brief
mock [format]	Behavioral, system design, case, panel, or technical mock
analyze	Paste any transcript — auto-detected format, scored + drilled
stories	Storybank: STAR + earned secrets + question mapping
decode	JD decoded through six lenses, fit verdict, recruiter questions
salary	Pre-offer comp scripting
negotiate	Offer negotiation strategy + exact scripts
progress	Score trends, gap analysis, next highest-leverage move

Validation

Passes python3 validate_plugins.py with 0 warnings ✅

Full system

For candidates who want persistent state across sessions and 23 total commands:
npx skills add dbhat93/job-search-os