feat: Add TOML build plugins and examples

Summary

Adds the first TOML-based Fabrik workflow with first-party plugin implementations and executable examples.

• Adds fabrik.toml manifest parsing alongside the existing Starlark path.
• Adds granular Rust build, test, vendor, and runtime integration through fabrik-rust.
• Adds an Apple/iOS simulator app plugin that can build and launch a Swift iOS app.
• Adds checked-in runtime task targets with cacheable and uncached execution modes.
• Adds resource-bounded scheduling primitives for local execution.
• Adds Rust and iOS example projects with Shellspec coverage.
• Adds docs for supported ecosystems, cache and execution behavior, tasks, and the agent-native product thesis.

Validation

Run locally on this branch:

• mise exec -- cargo check --workspace
• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test --workspace
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo build --release
• mise exec -- shellspec
• git diff --check

Also verified the iOS example can build and launch in an available simulator with xcrun simctl.

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