feat(core): remix published Square games into a new game

[feizhenhua]

Summary

Adds a native Remix tool (peer to the existing Publish tool) plus the wiring to make remix a default built-in skill for the Deeptide agent.

• RemixTool (crates/deeptide-core/src/tools/remix.rs): download the source of one or more published Paean Apps Square games by hash — resolve → POST /square/apps/:hash/remix → GET /v2/workspace/:ws/export/zip → zip-slip-safe extraction into .remix-sources/<hash>/. Scaffolds clide.json, LICENSE, .clideignore.
• Multi-parent remix graph in clide.json: tree-compatible remix.parent (mirrors the backend's single-parent field) and the full remix.parents[] DAG (each upstream with its borrowed role + revenue weight), so upstream creators can be credited.
• Wired as a default built-in: registry registration, the remix built-in skill prompt (dry-run → confirm → build → publish), the Remix wire-tool schema, permission lists (plan-blocked + confirm-by-default, matching Publish), and the discovery hint map.
• Docs: skills/publish and skills/remix playbooks + skills/README.md.

Accepted source forms: bare hashKey, hash.8x.gg, hash.clide.app, https://hash.clide.app/, https://8x.gg/hash, or hash=role (e.g. h1=gameplay h2=art h3=theme).

Security

• The Paean token stays in-process (bearer header only) — never logged or shell-exposed.
• Zip extraction uses enclosed_name() (rejects absolute paths / .. traversal).
• .remix-sources/ and clide.json are excluded from publishing.

Verification

• cargo build (full workspace) ✅
• cargo test -p deeptide-core — 556 lib + all integration tests pass (incl. built-in-skills catalog and tool-schema assertions) ✅
• cargo clippy -p deeptide-core — clean ✅
• Verified the committed branch builds + tests green in an isolated worktree (no unrelated working-tree edits).

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