Add Modal remote backend support

[thrashr888]

Summary

• add a live Modal remote backend adapter to the shared remote bridge
• wire Modal through backend/config plumbing and fix remote path persistence for workspace sync
• add Modal docs, templates, and runnable examples while fixing remote example bridge paths

Verification

• cargo test --quiet
• node --check scripts/remote-bridge.mjs
• live Modal smoke through agentkernel: create/start, managed /workspace sync, exec, snapshot, restore, restored exec

[Copilot]

Pull request overview

Adds a new hosted remote backend provider (“modal”) to agentkernel’s shared remote bridge system, wiring it through Rust backend/config plumbing and supplying templates/docs/examples for end-to-end usage.

Changes:

• Introduce modal as a first-class BackendType / RemoteProvider, including config schema and CLI help updates.
• Implement a live Modal adapter in scripts/remote-bridge.mjs (lifecycle, exec/attach, file APIs, workspace sync, snapshot/restore, endpoints).
• Add Modal template + example, and fix example bridge paths to be relative to their config locations.

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
templates/modal-remote.toml	Adds a built-in template for Modal remote sandboxes.
src/vmm.rs	Normalizes persisted work_dir/config_path for remote workspace sync and adds tests.
src/main.rs	Updates CLI help text to include modal as a backend option.
src/config.rs	Extends remote provider config to support Modal token fields and validates inline secrets.
src/backend/remote.rs	Adds RemoteProvider::Modal and maps Modal config into MODAL_* env vars for the bridge.
src/backend/mod.rs	Adds BackendType::Modal and wires it into remote sandbox creation/availability checks.
scripts/remote-bridge.mjs	Implements the Modal provider adapter and adds env aliasing + secret scrubbing for Modal tokens.
scripts/package.json	Adds the modal SDK dependency for the remote bridge.
scripts/package-lock.json	Locks Modal dependency tree (and records Node engine metadata).
examples/remote-runloop/agentkernel.toml	Fixes remote bridge path to be relative to the example config file.
examples/remote-e2b/agentkernel.toml	Fixes remote bridge path to be relative to the example config file.
examples/remote-daytona/agentkernel.toml	Fixes remote bridge path to be relative to the example config file.
examples/remote-modal/agentkernel.toml	Adds a runnable Modal example config.
examples/remote-modal/README.md	Adds runnable commands for the Modal example.
examples/README.md	Documents the Modal example alongside other remote examples.
docs/operations/remote.md	Documents Modal as a shipped live remote adapter with a config example.
docs/index.md	Adds Modal to the backend matrix and quickstart examples.
docs/config/toml.md	Documents [remote.modal] fields and token env configuration.
docs/config/backends.md	Adds modal to the remote backends list and notes SDK wiring.
docs/commands/templates.md	Adds modal-remote to the built-in templates list and usage examples.

Files not reviewed (1)

• scripts/package-lock.json: Language not supported

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[Copilot]

test_normalize_persisted_path_makes_relative_absolute compares the normalized path to current_dir().join(...), but normalize_persisted_path() may canonicalize and resolve symlinks (or return a different but equivalent absolute path). This can make the test flaky across platforms/setups. Consider canonicalizing the expected path too, or comparing via canonicalize()/ends_with rather than strict PathBuf equality.

Suggested change

	assert_eq!(
	std::path::PathBuf::from(normalized),
	current.join("examples/remote-modal")
	);
	let expected = std::fs::canonicalize(current.join("examples/remote-modal"))
	.unwrap_or_else(|_| current.join("examples/remote-modal"));
	assert_eq!(std::path::PathBuf::from(&normalized), expected);