Add mobile nav, /changelog, companyContext wiring, and @learnkit-ai/cli

[raj-khan]

Summary

• Mobile hamburger nav — Nav converted to 'use client'; hamburger button appears on mobile (≤720px), opens a slide-down menu with all nav links and the demo CTA. Closes on outside click or link navigation.
• /changelog page — Structured changelog at /changelog with typed ChangelogVersion[] data. Colour-coded Added/Changed/Fixed/Removed badges. Linked from the footer (replacing the Blog slot) and included in the sitemap.
• companyContext wiring — parseCompanyContext() in @learnkit-ai/core extracts stack, pace, and team-size signals from the free-text field. Context suffix is appended to all kind='project' lesson summaries. companyContext is now included in the stable seed, so different contexts produce different path IDs. 3 new tests (17 total).
• @learnkit-ai/cli — New package at packages/cli. npx @learnkit-ai/cli generate --role "PM" --tools "Claude,Cursor" --goal "ship faster" produces a pretty-printed 30-day path. --output json for machine-readable output. roles and tools commands list supported values. Zero external dependencies (uses node:util parseArgs). 3 tests.
• Manifest description — Updated from stale "workbench for teams that ship" copy to OSS-aligned description.

Test plan

• On mobile (≤720px): hamburger appears, tap opens menu, links navigate correctly, tap outside closes
• /changelog renders all 3 versions with correct badges and dates
• Demo: add company context "React team, ships weekly" → project lesson summaries mention React or weekly
• pnpm --filter @learnkit-ai/core test — 17 tests pass
• pnpm --filter @learnkit-ai/cli test — 3 tests pass
• pnpm --filter @learnkit-ai/web typecheck — clean
• pnpm --filter @learnkit-ai/cli typecheck — clean