chore: remove plugin system divergence with upstream

[randomm]

What

Restore upstream's complete plugin system and remove our simplified fork variant.

Why

Our fork stripped out 6 plugin files (~1505 lines) to simplify the plugin loading system. This divergence:

• Is NOT tracked in .fork-features/manifest.json
• Provides no value to our fork (removed plugins are for Poe/Cloudflare which we don't use)
• Causes pain during every upstream sync (merge conflicts, manual reconciliation)
• Violates our principle: "Keep divergence only in areas where there is value"

Scope

Restore these deleted files:

• packages/opencode/src/plugin/cloudflare.ts (67 lines)
• packages/opencode/src/plugin/install.ts (439 lines)
• packages/opencode/src/plugin/loader.ts (174 lines)
• packages/opencode/src/plugin/meta.ts (188 lines)
• packages/opencode/src/plugin/shared.ts (323 lines)
• packages/opencode/src/plugin/github-copilot/models.ts (144 lines)

Restore directory structure:

• packages/opencode/src/plugin/github-copilot/ directory with copilot.ts and models.ts

Update these files to match upstream:

• packages/opencode/src/plugin/index.ts (currently 138 lines vs upstream ~290)
• packages/opencode/src/plugin/codex.ts
• packages/opencode/src/plugin/copilot.ts

No dependency changes required:

• Neither fork uses effect as a direct package.json dependency
• All required packages already present

Acceptance Criteria

• All 6 deleted files restored from upstream anomalyco/dev
• Plugin directory structure matches upstream (9 files total)
• plugin/index.ts restored to upstream version
• All existing auth flows still work (Codex, Copilot, GitLab)
• bun test passes (plugin-related tests)
• .fork-features/manifest.json does NOT need an entry (divergence removed, not added)
• git diff anomalyco/dev -- packages/opencode/src/plugin/ shows minimal/no differences

Out of Scope

• Adding new plugins (Poe, Cloudflare) — we're just restoring infrastructure, not enabling unused features
• Changing plugin loading behavior — should match upstream exactly
• User-facing changes — this is internal refactoring

Definition of Done

• Code restored and matches upstream
• Tests passing
• No new fork divergence introduced
• Upstream sync pain point eliminated for this module