docs(readme): lead with the agent landing line; the new editor replaces Re-Studio

[NiJingzhe]

What

Both READMEs (English and 中文) rewritten around the new editor, with the agent landing line first.

• First thing a visitor sees: the line to paste to a coding agent, Read physpace.dev/sca/landing.md and landing me on this sdk., right under the title, with two sentences on what it does.
• Pillar 3 is now the code-first editor (sca edit) with a new screenshot (img/capability/editor.png: a picked face, the generated selection cell, its properties, the log dock). The Re-Studio MVP pillar is gone.
• Re-Studio removed everywhere: the standalone sca-web-editor / Re-mode references, the studio workflow link, the demo gif/mp4 (10 MB) and the skill workflow reverse-engineering-studio.md (plus its pointer in step-reconstruction.md). STEP reconstruction points at the step-reconstruction workflow.
• Update notes rewritten for 2.2 (editor, one runtime and cache, landing path); stale "current release 2.1.3" replaced; install section mentions the editor extra, uv tool / pipx, and sca example.
• CHANGELOG: a "Removed" entry.

Check

• Every relative link and image in both READMEs resolves.
• test_skill_build, test_skill_cli: 44 passed.

Note

PyPI's 2.2.0b1 (uploaded 2026-10-02) has no editor, so the editor cannot ship under that version number; the README is version-neutral on purpose. The release version still has to be chosen.