docs: README pass — the harness axis (opencode open, telaio by subscription)

[aarontrowbridge]

Problem — The README predates the harness reframe (spec addendum 2026-08-17): it calls opencode a 'vendored chat harness' rather than one of switchable harnesses, and never positions telaio. Post-#389 the open harness is also plan-first, which the README doesn't say.

Acceptance Criteria

• The README names the harness axis: opencode (open, plan-first) and telaio (subscription, arriving), and that the studio is harness-agnostic
• The commercial split is honest: studio + open harness open; telaio and -issimo tiers entitled
• No feature claims beyond what ships (telaio marked as arriving)