Add E2B sandbox provider plugin (#4452)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Sandbox environments are part of that execution layer, and the
recent core refactor moved provider-specific behavior to a generic
plugin seam
> - This pull request adds a dedicated `@paperclipai/plugin-e2b` package
so E2B can live entirely outside core host code
> - Because the feature is still unreleased, the plugin should model
third-party packaging directly instead of carrying extra
backward-compatibility complexity in core or the workspace lockfile
> - This branch therefore makes the E2B provider a standalone
publishable package, documents the package-local dev flow, and keeps the
publish manifest/runtime dependency story correct
> - The benefit is that E2B becomes a true plugin reference
implementation that can be installed by package name without reopening
core Paperclip code

## What Changed

- Added `packages/plugins/paperclip-plugin-e2b` as the E2B sandbox
provider plugin package
- Implemented config validation, lease acquire/resume/release/destroy
handlers, workspace realization, and command execution for E2B sandboxes
- Excluded the E2B plugin package from the root workspace so the repo no
longer needs `pnpm-lock.yaml` churn for its third-party dependency graph
- Added package-local development/install support plus a prepack
manifest generator so the published tarball still declares
`@paperclipai/plugin-sdk` and `e2b` runtime dependencies
- Addressed review feedback by fixing sandbox cleanup on acquire
failures, rejecting blank templates, normalizing fractional `timeoutMs`,
and always passing the configured template name to the E2B SDK
- Updated focused Vitest coverage for config normalization, validation,
acquire cleanup, command execution, and lease release behavior
- Updated the Dockerfile deps stage to copy the E2B package manifest so
the policy check stays in sync

## Verification

- `cd packages/plugins/paperclip-plugin-e2b && pnpm install
--ignore-workspace --no-lockfile`
- `cd packages/plugins/paperclip-plugin-e2b && pnpm build`
- `cd packages/plugins/paperclip-plugin-e2b && pnpm --ignore-workspace
test`
- `cd packages/plugins/paperclip-plugin-e2b && pnpm --ignore-workspace
typecheck`
- `cd packages/plugins/paperclip-plugin-e2b && npm pack --dry-run`

## Risks

- The package now relies on a prepack manifest rewrite so the
publish-time dependency list stays correct while the repo-local dev
manifest stays workspace-light
- The current repo snapshot is still unreleased, so the generated
publish manifest points at the repo SDK version until the normal release
flow rewrites versions before publish
- Real-world E2B environments may still expose edge cases around
lifecycle timing or sandbox metadata beyond the mocked unit coverage

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex via `codex_local`
- Model ID: `gpt-5.4`
- Reasoning effort: `high`
- Context window observed in runtime session metadata: `258400` tokens
- Capabilities used: terminal tool execution, git, GitHub CLI, and local
build/test inspection

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

scripts/link-plugin-dev-sdk.mjs

@@ -0,0 +1,35 @@
+#!/usr/bin/env node
+
+import { existsSync, mkdirSync, lstatSync, rmSync, symlinkSync } from "node:fs";
+import { dirname, join, relative, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const repoRoot = resolve(__dirname, "..");
+const packageDir = process.cwd();
+const sdkDir = join(repoRoot, "packages", "plugins", "sdk");
+const scopeDir = join(packageDir, "node_modules", "@paperclipai");
+const linkTarget = join(scopeDir, "plugin-sdk");
+
+if (!existsSync(join(packageDir, "package.json"))) {
+  throw new Error(`No package.json found in plugin directory: ${packageDir}`);
+}
+
+mkdirSync(scopeDir, { recursive: true });
+
+try {
+  const stat = lstatSync(linkTarget);
+  if (stat.isSymbolicLink()) {
+    rmSync(linkTarget, { force: true });
+  } else {
+    console.log("  i Keeping existing installed @paperclipai/plugin-sdk directory in place");
+    process.exit(0);
+  }
+} catch {
+  // target does not exist yet
+}
+
+const relativeSdkDir = relative(scopeDir, sdkDir);
+symlinkSync(relativeSdkDir, linkTarget, "dir");
+
+console.log(`  ✓ Linked local @paperclipai/plugin-sdk for ${packageDir}`);