Add level picker, 4 new role curricula, ROADMAP, and blog posts

[raj-khan]

Summary

• Demo: level + company context — Added a 3-option level picker (Beginner / Intermediate / Advanced) and an optional "Add company context" textarea to step 3 of the demo flow. Both values are passed to generateLearningPath() — level affects lesson pacing; companyContext passes through to the schema (used in v1).
• Role curricula: Marketer, Founder, Operations, Researcher — All 8 supported roles now have role-specific 4-week lesson content. Previously these 4 fell back to the generic curriculum.
• Tests — Added 5 new tests in generate.test.ts: it.each for all 4 new roles (valid path + correct weeks), and level pacing test for all new roles.
• ROADMAP.md — Added root-level roadmap: v0 (done), v1 (planned), v2 (exploratory), and explicit "will never be" section.
• Blog posts (4 new) — CSS theming deep-dive, role curriculum design decisions, companyContext field explainer, and a theming post. Blog now has 10 posts total.

Test plan

• /demo — step 3 shows level picker (3 pills); "Add company context" link reveals textarea; curriculum changes minutes when level is switched
• /demo — Marketer, Founder, Operations, Researcher each produce role-specific lesson titles in step 5 (not the generic "Your first system prompt" sequence)
• pnpm --filter @learnkit-ai/core test — 14 tests pass
• pnpm --filter @learnkit-ai/web typecheck — clean
• /blog — shows 10 posts, new posts render at their slugs
• ROADMAP.md renders correctly on GitHub