Strip language surfaces from hyperpush root

[snowdamiz]

Summary

Strip the copied language/toolchain tree out of the product repo so the root is genuinely product-only again.

This keeps:

• mesher/
• mesher/landing/
• mesher/frontend-exp/
• minimal product-root docs / policy / CI / verifier surfaces

and removes the copied Mesh language/compiler/docs/registry/packages-site tree that does not belong in the product repo.

What changed

• deleted copied language-owned roots from the product repo:
  • compiler/
  • website/
  • packages-website/
  • registry/
  • examples/
  • tests/
  • tools/
  • benchmarks/
  • articles/
  • mesh-slug/
  • copied .gsd/, .cargo/, .githooks/, and the language release/workflow/verifier stack
• kept product-owned root surfaces and rewrote them around the new truth:
  • README.md
  • CONTRIBUTING.md
  • SUPPORT.md
  • .github/CODEOWNERS
  • .github/ISSUE_TEMPLATE/*
  • .github/dependabot.yml
  • .gitignore
• added product CI:
  • .github/workflows/ci.yml
  • checks out sibling mesh-lang, builds meshc, runs the Mesher maintainer verifier, and builds both mesher/landing and mesher/frontend-exp
• kept landing deploy workflow only for landing:
  • .github/workflows/deploy-landing.yml
• rewrote the Mesher maintainer proof surface to be product-only instead of depending on copied compiler tests:
  • mesher/scripts/verify-maintainer-surface.sh
  • scripts/verify-m051-s01.sh
  • mesher/README.md
  • mesher/.env.example
• explicitly kept frontend-exp product-owned under:
  • mesher/frontend-exp/
• removed stale tracked generated product artifacts:
  • mesher/mesher
  • mesher/mesher.ll
  • mesher/output
  • tracked landing tmp banner output / tsbuildinfo

Explicit ownership note

frontend-exp stays with the product repo.

This branch keeps it at:

• mesher/frontend-exp/

It does not move back into mesh-lang.

Verification

• bash scripts/verify-landing-surface.sh
• bash scripts/verify-m051-s01.sh
• npm --prefix mesher/landing ci && npm --prefix mesher/landing run build
• npm --prefix mesher/frontend-exp ci && npm --prefix mesher/frontend-exp run build

Notes

• GitHub redirected the push target from hyperpush-org/hyperpush-mono to hyperpush-org/hyperpush. This PR is opened against the redirected repo.
• This is the product-repo half of the cleanup. If this merges, the mesh-lang split-boundary wrappers may need a follow-up to match the slimmer product-root bundle/CI contract exactly.