fix(harness_manager): support Python 3.9 (closes #27)

Fixes the immediate crash @WhoLsJohnGalt reported: every brew user on
macOS-default Python 3.9 hits

    File ".../harness_manager/schema.py", line 190, in <module>
        def validate(manifest_path: Path | str) -> dict:
    TypeError: unsupported operand type(s) for |: 'type' and 'type'

immediately after `brew install agentic-stack`. PEP 604 union syntax
(`Path | str`) requires Python 3.10+ at runtime; the macOS Command Line
Tools default is still 3.9.

There are 25 such annotations across 8 files in `harness_manager/`. Rather
than rewrite each to `Union[Path, str]` / `Optional[X]`, this commit adds
`from __future__ import annotations` (PEP 563) to each affected file so
all annotations are stored as strings and never evaluated at import time.
Works on Python 3.7+, which covers every macOS-shipped Python in the wild.

Files changed (all `harness_manager/`):
  schema.py state.py install.py remove.py post_install.py
  manage_tui.py doctor.py status.py

Audited the rest of the repo: no PEP 604 syntax in `.agent/`,
`onboard*.py`, or any other Python touchpoint, so no other module needs
the same treatment.

The reporter's suggested fix (declare `python@3.10` as a brew dep + pin
the binary in install.sh) would also work but pulls in ~150 MB on every
brew install and conflicts with users who manage their own python (pyenv,
asdf). The future-annotations approach is zero-cost and benefits non-brew
users (Linux, manual clone) on system Python 3.9 too.

Author: codejunkie99

harness_manager/doctor.py

@@ -8,6 +8,8 @@
 from filesystem signals and ASKS before synthesizing — never silently
 writes. Codex's UX framing: doctor must not mutate without consent.
 """
+from __future__ import annotations
+
 import os
 import shutil
 import sys

harness_manager/install.py

@@ -4,6 +4,8 @@
 post_install actions to a target project, records what was done in
 .agent/install.json. install.sh and install.ps1 dispatch into here.
 """
+from __future__ import annotations
+
 import shutil
 import sys
 from pathlib import Path

harness_manager/manage_tui.py

@@ -9,6 +9,8 @@
 for consistency with the existing wizard. No new UI primitives
 beyond the multi-select widget added at the same time.
 """
+from __future__ import annotations
+
 import os
 import shutil
 import signal

harness_manager/post_install.py

@@ -8,6 +8,8 @@
 codex review of the v1.0 vision plan flagged generalized run_command as
 DSL creep; named built-ins are the constrained alternative.
 """
+from __future__ import annotations
+
 import hashlib
 import os
 import platform

harness_manager/remove.py

@@ -4,6 +4,8 @@
 quarantine — codex UX framing: don't over-help, no undo machinery
 to learn or trust). User runs `git reset` if they want recovery.
 """
+from __future__ import annotations
+
 import shutil
 import sys
 from pathlib import Path

harness_manager/schema.py

@@ -5,6 +5,8 @@
 python3 with no pip step. A 50-line recursive checker is cheaper than
 adding a `jsonschema` dependency.
 """
+from __future__ import annotations
+
 import json
 from pathlib import Path
 from typing import Any

harness_manager/state.py

@@ -6,6 +6,8 @@
 the _episodic_io flock pattern is the wrong abstraction for install.json
 because of read-modify-write — this module uses the right one.
 """
+from __future__ import annotations
+
 import json
 import os
 import tempfile

harness_manager/status.py

@@ -3,6 +3,8 @@
 Same content as the manage menu's header pane in the v1.0 vision —
 shipped here as a discrete subcommand instead of a TUI element.
 """
+from __future__ import annotations
+
 from pathlib import Path
 from typing import Callable