Remove legacy shell CLI assets and stale docs unrelated to the Go oc CLI

[sven1103-agent]

The repository still contains a large amount of legacy opencode-helper shell-era code and documentation that is unrelated to the current Go CLI (oc). This makes the codebase harder to navigate, creates ambiguity about the supported product surface, and risks contributors wiring new work to deprecated assets.

Targets to review and remove or rewrite:

• scripts/opencode-helper
  Legacy shell CLI entrypoint for the deprecated helper CLI.
• scripts/opencode-helper-install
  Legacy installer for the shell helper CLI.
• scripts/release/install.sh
  Bootstrap script for downloading and verifying opencode-helper-install.
• scripts/release/build-opencode-helper-bundle.sh
  Release bundling flow centered on opencode-helper release artifacts.
• scripts/test-opencode-helper-install
  Smoke tests for the deprecated shell installer flow.
• scripts/test-release-opencode-helper-bundle
  Smoke tests for the deprecated shell release bundle.
• docs/opencode-helper-cli.md
  Large planning/spec document still centered on the helper CLI, shell installer, self-update flow, and legacy command surface.
• docs/opencode-helper-go-migration-milestones.md
  Migration plan that still describes the transition from the old helper CLI rather than the current supported product state.
• docs/opencode-helper-v2-milestones.md
  Milestone plan still expressed in legacy opencode-helper terms.
• README sections that still describe legacy helper/install/update flows and shell-era command names such as opencode-helper-install, self-update, oc init, and oc preset use where those are no longer the intended product surface.
• cmd/init.go and cmd/preset.go
  Legacy bundled-preset/V1 command surface that should be re-evaluated against the current Go CLI direction and removed if no longer supported.
• cmd/init_test.go and cmd/preset_test.go
  Tests tied to the legacy init/preset flows.

Additional cleanup candidates to review:

• internal/source/source.go
  Still persists registry state under an opencode-helper path, which looks like leftover naming from the legacy product.
• Any remaining references to opencode-helper, opencode-helper-install, shell release bundles, or self-update across docs/workflows/templates should be audited and either migrated to oc terminology or deleted.

Suggested cleanup approach:

1. Define the canonical supported product surface for the Go CLI (oc).
2. Remove deprecated shell scripts and their tests once no supported path depends on them.
3. Rewrite or delete stale planning/docs pages that describe the old helper CLI.
4. Update README and issue templates so they only describe current supported flows.
5. Rename lingering config/storage paths that still use legacy opencode-helper naming if backward compatibility is not required.

Done criteria:

• no repo docs present the deprecated shell helper as a current product path
• no legacy shell release/install scripts remain unless explicitly retained for backward compatibility
• command docs align with the current Go CLI only
• contributor-facing repo structure is clearly centered on oc