[codex] add sysroot and prefix execution roots

[veighnsche]

Closes #11

What changed

• adds first-class --sysroot and --prefix globals to the recipe CLI
• threads sysroot/prefix execution roots through engine and lifecycle paths
• maps absolute install helper paths under SYSROOT while keeping BUILD_DIR and tool-prefix paths as explicit host-side passthrough roots
• rejects . / .. traversal and validates against sysroot escape through existing ancestors
• adds CLI and helper regression coverage for sysroot-aware installs
• updates the current README.md surface to match the implementation

Why

recipe bootstrap and the A/B install story were blocked on a missing canonical target-root model. This change lands that foundation in the actual owner instead of adding a shallow bootstrap wrapper on top of host-path assumptions.

Impact

Recipes now receive SYSROOT and PREFIX as standard scope constants. Absolute filesystem helper paths resolve under the chosen target sysroot by default, which makes offline, chroot, and A/B composition possible without embedding mount paths into recipes.

Validation

• cargo test --manifest-path tools/recipe/Cargo.toml --tests