feat(permissions): LP4-1/LP4-2 — enforcer Landlock OS + consommation PTY du plan

LP4-1 : adaptateurs SandboxEnforcer concrets (LandlockSandbox Linux via le LSM
Landlock, NoopSandbox ailleurs) + default_enforcer() cfg-sélectionné, prêts à
être injectés au composition root (LP4-3). Aucun changement de comportement
runtime tant que rien n'est câblé : SpawnSpec.sandbox reste None.

LP4-2 : PortablePtyAdapter consomme SpawnSpec.sandbox via spawn_command_sandboxed
(thread jetable restreint par enforce() puis fork → le domaine Landlock est hérité
par l'enfant à travers execve ; les autres threads d'IdeA ne sont jamais sandboxés).
Builder additif with_sandbox_enforcer ; fail-closed si enforce() échoue.

Tests : domain + infrastructure verts (dont les tests Landlock réels de fencing
read-only/write-only et la confine-au-thread).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -979,6 +979,26 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "34aa73646ffb006b8f5147f3dc182bd4bcb190227ce861fc4a4844bf8e3cb2c0"
 
+[[package]]
+name = "enumflags2"
+version = "0.7.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1027f7680c853e056ebcec683615fb6fbbc07dbaa13b4d5d9442b146ded4ecef"
+dependencies = [
+ "enumflags2_derive",
+]
+
+[[package]]
+name = "enumflags2_derive"
+version = "0.7.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "67c78a4d8fdf9953a5c9d458f9efe940fd97a0cab0941c075a813ac594733827"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -1923,6 +1943,7 @@ dependencies = [
  "domain",
  "fastembed",
  "git2",
+ "landlock",
  "notify",
  "portable-pty",
  "reqwest 0.12.28",
@@ -2131,6 +2152,17 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "landlock"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "635839550ae8b90d9fd2571460a6645dc0aec070225956ca7a2831ed31d2795d"
+dependencies = [
+ "enumflags2",
+ "libc",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "lazy_static"
 version = "1.5.0"