Implement Landlock sandbox runner

[Eilen6316]

Goal

Add a Linux Landlock-backed sandbox runner that can enforce filesystem boundaries for safe local execution profiles.

Context

v4.1.0 added docs/design/sandbox-landlock.md as the design source. This issue tracks the implementation work after the design-only release.

Scope

• Add a landlock runner kind and config parsing.
• Implement capability probing for Linux, Landlock syscall availability, ABI version, and ruleset application.
• Enforce a minimal read_only profile first.
• Extend to workspace_write only after read-only behavior is stable.
• Preserve current noop, local, and bubblewrap behavior.
• Fail closed for safe profiles when Landlock cannot enforce the requested boundary.
• Record capability probe and enforcement state in audit/sandbox metadata.

Non-goals

• Do not add network isolation through Landlock; Landlock is filesystem-only.
• Do not replace HITL, policy, audit, or bubblewrap.
• Do not make Landlock the default until the compatibility matrix is proven.

Acceptance Criteria

• make sandbox, make security, and make test pass.
• Unsupported kernels return a clear unavailable reason without crashing.
• read_only allows reads under configured roots and blocks writes.
• Missing or symlink-escaped allowed roots fail closed for safe profiles.
• Documentation is updated with the implemented behavior and remaining limitations.