feat(plugins): add Modal sandbox provider plugin (#6245)

## Thinking Path

> - Paperclip orchestrates AI agents through company-scoped
control-plane workflows and extensible runtime integrations.
> - Sandbox providers are part of that extension surface because they
let agents execute isolated work without baking each provider into the
core server.
> - Modal already offers managed sandboxes with filesystem, process,
timeout, and networking controls that map onto Paperclip's sandbox
provider contract.
> - The repo did not have a Modal provider plugin, so teams wanting
Modal-backed sandboxes had no first-party integration path.
> - This pull request adds a standalone
`packages/plugins/sandbox-providers/modal` plugin that implements the
provider contract, worker entrypoint, docs, and tests.
> - The benefit is that Modal can now be installed as a provider plugin
without expanding the core control-plane surface area.

## What Changed

- Added a new `packages/plugins/sandbox-providers/modal` package with
the plugin manifest, worker entrypoint, and exported plugin surface.
- Implemented Modal-backed sandbox lifecycle support for creation,
command execution, file operations, networking options, termination, and
metadata translation.
- Added focused Vitest coverage for config validation, env handling,
lifecycle flows, networking behavior, and error mapping.
- Documented installation, configuration, and usage requirements in the
plugin README.
- Removed misleading `MODAL_TOKEN_*` fallback behavior so authentication
relies on supported Modal credentials only.

## Verification

- `pnpm -r typecheck`
- `pnpm test:run`
- `pnpm build`
- `cd packages/plugins/sandbox-providers/modal && pnpm test`

## Risks

- Low to medium risk: this is isolated to a new plugin package, but
runtime behavior still depends on live Modal account credentials and
service-side sandbox semantics.
- Modal's current docs target a newer Node baseline than the repo
default, so the first live install should confirm credential loading and
sandbox startup behavior in a real Modal workspace.
- No UI or schema changes are included in this PR.

> 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 Paperclip `codex_local` agent (GPT-5-class Codex
coding model; exact backend model ID is not exposed by the runtime),
with tool use, shell execution, and code-editing capabilities enabled.

## 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
- [x] 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

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

scripts/release-package-manifest.json

@@ -114,6 +114,11 @@
     "name": "@paperclipai/plugin-e2b",
     "publishFromCi": true
   },
+  {
+    "dir": "packages/plugins/sandbox-providers/modal",
+    "name": "@paperclipai/plugin-modal",
+    "publishFromCi": false
+  },
   {
     "dir": "ui",
     "name": "@paperclipai/ui",