fix(sandbox): strict profile no longer lets a child rewrite ~/.kigi

Port of upstream grok-build 1.0.14 ("--sandbox strict now restricts
writes to ~/.grok/sessions only"), shaped for Kigi.

The strict profile granted the whole ~/.kigi read-write, so a sandboxed
command could plant a global SessionStart hook or flip
`[sandbox].profile` in config.toml for the next launch. Landlock cannot
carve a child out of a write grant, so the home is now read-only
except an explicit allowlist of state subtrees (run, sessions,
worktrees, worktree_pool, tasks, memory, logs, debug, crash, memtrace,
trace-exports); config.toml, auth.json, sandbox.toml, hooks/, bin/ and
vendor/ stay read-only. A symlinked state dir under the home is
followed only to the default ~/.kigi/sessions.

What the process itself writes there after enforcement moves or is
minted first:
- kigi-shell `apply_sandbox` mints the device id, extracts the bundled
  rg and the bundled skills BEFORE enforcement for every profile that
  restricts home writes (`restricts_home_writes`, custom profiles
  extending strict included).
- The sandbox audit log lives at ~/.kigi/sessions/sandbox-events.jsonl.
- Leader lock/socket default to ~/.kigi/run/, the leader stderr log to
  ~/.kigi/logs/leader.log; discovery scans run/ and the legacy root,
  and the default target prefers the current socket.
Root-level caches (version.json, agent_id, slash-mru.json, pager.toml,
claude_import_state.json) degrade best-effort under strict; documented.

Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4

crates/codegen/kigi-sandbox/src/logging.rs

@@ -1,7 +1,7 @@
 //! Sandbox event logger.
 //!
 //! Events (profile applied, violations, bypasses) are buffered in memory and
-//! flushed as JSONL to `~/.kigi/sandbox-events.jsonl`.
+//! flushed as JSONL to `~/.kigi/sessions/sandbox-events.jsonl`.
 
 use std::path::PathBuf;
 use std::sync::Mutex;
@@ -87,8 +87,12 @@ impl SandboxLogger {
         Ok(())
     }
 
+    /// Under `sessions/`: the only home subtree a strict sandbox leaves
+    /// writable, and this flush runs after enforcement.
     fn log_file_path() -> PathBuf {
-        kigi_config::kigi_home().join("sandbox-events.jsonl")
+        kigi_config::kigi_home()
+            .join("sessions")
+            .join("sandbox-events.jsonl")
     }
 }