9harness: the harness fs daemon

The last item of the 9P plan, replacing zmxify's introspection half: a
read-only, fresh-from-disk 9P view of every agent harness's state on
this machine, posted as `harness` like any other service, so a shell
inside a 9ns --mntgen mount reads it at /mnt/9p/harness with no setup.

  /pid /uptime   /claude/{projects,history,skills}
  /codex/{sessions,session-index,history}
  /omp /hermes /dsh     the mirrors
  /skills/{claude,codex,omp}

Nothing is cached: a lookup, getattr or readdir walks the real
filesystem, so a transcript grows as its harness writes it and a new
session appears as soon as its file lands. Writes answer EPERM, and no
name that looks like a credential, key, token or auth store is ever
answered at any depth.

Three findings from the adversarial pass, each with its regression:

- The read path composed <base>/<rel> and opened it in one call, which
  follows symlinks. A name swapped for a link between the walk and the
  read served bytes from outside every pinned root (proved against
  /etc/passwd). Every stat, read and readdir now resolves through
  openIn, which walks from the base one component at a time with
  O_NOFOLLOW, and O_PATH for the intermediates, so no component can
  redirect the walk. O_PATH also keeps a fifo in a root from parking the
  daemon in open(); a read refuses anything but a regular file.
- Joining a child onto an empty relative path returned an uncopied
  scratch slice, so every file at the top of a mirror root (/hermes/x,
  /dsh/x) listed but read back uninitialized stack bytes.
- A directory past the comptime caps was served short, and a short
  listing cannot be told from a small directory. The caps answer NFILE
  now. Staging also stops at the first record that does not fit instead
  of packing a shorter one behind it, which dropped that entry from the
  listing across the read boundary.

Suites: 13/13 unit (fake HOME, never the live roots), 36/0 end-to-end
including the mntgen money shot and the live ~/.claude/.credentials.json
proved unreachable, 131/131 programs-test.

1 files changed, 20 insertions, 0 deletions

diff --git a/build.zig b/build.zig
index 334da9f..795d8e2 100644
--- a/build.zig
+++ b/build.zig
@@ -156,6 +156,7 @@ pub fn build(b: *std.Build) void {
     const is_linux = target.result.os.tag == .linux;
     const want_9proc = b.option(bool, "9proc", "Build the 9proc library module and demo (default: target is Linux)") orelse is_linux;
     const want_9ns = b.option(bool, "9ns", "Build the 9ns FUSE mount CLI (default: target is Linux; Linux only)") orelse is_linux;
+    const want_9harness = b.option(bool, "9harness", "Build the harness fs daemon (default: target is Linux; Linux only)") orelse is_linux;
     if (want_9ns and !is_linux) {
         std.debug.print("error: -D9ns=true needs a Linux target (got {s})\n", .{@tagName(target.result.os.tag)});
         std.process.exit(1);
@@ -206,6 +207,25 @@ pub fn build(b: *std.Build) void {
         programs_itest.dependOn(p.itest_step);
     }
 
+    // The harness fs daemon: a read-only 9P view of every harness's
+    // state, posted by default as `harness` (Linux only: it posts into
+    // $XDG_RUNTIME_DIR/9p through cloud9.post).
+    if (want_9harness and !is_linux) {
+        std.debug.print("error: -D9harness=true needs a Linux target (got {s})\n", .{@tagName(target.result.os.tag)});
+        std.process.exit(1);
+    }
+    const harness_build = @import("9harness/build.zig");
+    const harness: ?harness_build.Artifacts = if (want_9harness) harness_build.add(b, .{
+        .target = target,
+        .optimize = optimize,
+        .cloud9 = module,
+        .ns = if (ns) |p| p.exe else null,
+    }) else null;
+    if (harness) |p| {
+        programs_test.dependOn(p.test_step);
+        programs_itest.dependOn(p.itest_step);
+    }
+
     // 9proc's end-to-end suites drive the demo through a 9ns mount,
     // so they are wired once both fragments have run.
     if (proc) |i| {