session-wrapper: remove built-in allowlist and enforce TACACS-controlled command authorization policy

[Rod-Persky]

Parent epic: #152

session-wrapper: remove built-in allowlist and enforce TACACS-controlled command authorization policy

Objective

Remove the built-in/local command allowlist fast path and replace it with TACACS+-controlled per-exec authorization behavior inside the Landlock boundary.

Breaking changes are allowed. Removing CLI flags, files, and tests tied to the old proof-of-concept allowlist is expected.

Background

The existing built-in allowlist is platform-dependent and creates a local authorization bypass. In the new model, Landlock is the maximum execution boundary and TACACS+ remains the dynamic per-command decision point inside that boundary.

Required Behavior

• Delete the unconditional built-in exec allowlist, including builtin_allowlist.txt.
• Remove or repurpose production --allowlist behavior. A production local file must not bypass TACACS+ command authorization.
• Initial production behavior is:

landlock-command-auth=required

• Every intercepted exec inside the Landlock profile still receives per-exec TACACS+ authorization.
• The per-command authorization request remains:

service=shell
cmd=/path/to/executable
cmd-arg=...

• Unsupported future optimization modes such as profile-only or landlock-auth-cache should be rejected, ignored safely, or left unimplemented with explicit errors.

Security Requirements

• Winning the seccomp user-notification TOCTOU race must not permit execution outside the Landlock profile.
• A local path list must not silently permit commands without TACACS+ visibility.
• Any future command authorization cache must be explicit TACACS+ profile data, not compiled-in behavior.

Acceptance Criteria

• Built-in allowlist code and data are removed from production paths.
• Tests prove landlock-command-auth=required causes per-exec TACACS+ authorization for every intercepted exec.
• Tests prove local allowlist fast paths are not used in production profile mode.
• CLI/help/docs no longer imply that local allowlists provide production authorization bypasses.