feat(review-tutor): always register the Claude Code and Codex connectors

[ElbertePlinio]

Closes #63 — enables the harness connectors and removes the reviewTutorHarnessConnectors flag in one step.

Why one step

Review Tutor is private and installed from a checkout (pi install <path>), so main is the build; a separate "enabled" commit would guard nothing between two merges. Elberte tested the flag-on build (identical code) before #76–#80 merged.

What changed

• createConnectorRegistry always registers Pi, Claude Code, Codex (that order); src/flags.ts and the REVIEW_TUTOR_FLAGS switch are gone.
• A connector only appears in the Harness picker when its executable is found and supported at startup; the helper line explains the rest (unchanged behaviour, now unconditional).
• Tests: flag on/off cases collapsed; every registry built in tests injects which/execFile stubs so discovery never spawns real binaries (PATH-shim proof: 0 calls across the suite).
• README: flag paragraph replaced.

Tested

• vitest run packages/review-tutor/test — 344 passed; package typecheck clean; root bun run check green.
• PATH shim for claude/codex during the suite — 0 invocations.

Not tested

• No new real-provider smoke (behaviour identical to the flag-on build Elberte tested on 2026-08-26).

Risk class: standard (one reviewer).