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.

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