docs: remove legacy Claude and OpenSpec repository artifacts

[LeXwDeX]

Closes #477

Why

This repository no longer keeps Claude-specific or OpenSpec repository artifacts. The root CLAUDE.md deletion is intentional and is delivered with the repair wave rather than restored or treated as accidental worktree damage. The Git index contains no tracked OpenSpec-named path, so the absence is verified instead of inventing a deletion.

What changed

• Deleted the tracked root CLAUDE.md (288 lines of legacy Claude-specific instructions).
• Verified /openspec/ is already ignored (.gitignore:41) and that git ls-files shows no tracked OpenSpec workspace/config artifact — no invented deletion.
• Delivery binding record (.specgit.yaml) updated in a separate chore commit (harness bookkeeping, not product diff).
• Retained AGENTS.md hierarchy remains the repository instruction discovery path; runtime code, tests, CI, policy, generated files, and historical prose references are unchanged.

Evidence

• Delivery branch: docs/477-remove-legacy-artifacts (cut from origin/main a3669c3; isolated worktree, user checkout untouched).
• Product commit: 7c371fe9c0 — product diff is exactly one file: CLAUDE.md deleted (288 deletions, no other paths).
• git ls-files | grep -i openspec → no matches (exit 1) at commit 7c371fe.
• git check-ignore -v openspec/ → .gitignore:41:/openspec/ (ignored as required).
• Tracked claude-named runtime artifacts (docs page, llm fixtures, import template, ui icons) deliberately left untouched per issue scope.

Checklist

• Root CLAUDE.md removed in a dedicated delivery
• git ls-files confirms no tracked OpenSpec workspace/config artifact remains
• Runtime code, tests, CI policy, generated files, and unrelated documentation unchanged
• Repository instruction discovery still uses the retained AGENTS.md hierarchy
• Required PR checks pass and specgit finish --json exits 0 (pending — checks to run; not finished/merged in this wave step)