feat(windows-install): bundle portable MinGit instead of relying on winget

User hit a real failure case: their system Git was in a half-installed state
(can neither uninstall nor reinstall) and winget refused to work around it.
We were one step away from shipping an installer that would have left users
with exactly the problem he already had.

What other agents do (reality check):
- Claude Code: requires pre-installed Git; breaks if user doesn't have it.
- OpenCode, Codex: don't need bash at all — PowerShell-first design.
- Cline: uses whatever shell VSCode is configured with; installs nothing.

None of them solve the "broken system Git" problem.  We need to own our Git.

Changes:
- scripts/install.ps1::Install-Git: dropped winget path entirely.  Now:
  (1) use existing git if present; (2) download portable MinGit from the
  official git-for-windows GitHub release to %LOCALAPPDATA%\hermes\git.
  No winget, no admin, no Windows installer registry, no system impact.
- Added %LOCALAPPDATA%\hermes\git\{cmd,usr\bin} to User PATH so git + bash
  + POSIX coreutils (which, env, grep, …) resolve in fresh shells.
- tools/environments/local.py::_find_bash: reorder so Hermes' portable
  MinGit install is checked BEFORE falling through to shutil.which("bash")
  or system install locations.  This way a broken system Git can't
  hijack the bash lookup.
- README + installation docs reworded to reflect the new story: "portable
  Git Bash, isolated from any system install, recoverable via rm -rf if it
  ever breaks."

Recoverability: if Hermes' Git install ever breaks, ``Remove-Item %LOCALAPPDATA%\hermes\git``
and re-run the installer — no system impact, no uninstall drama, no winget
to fight with.

tools/environments/local.py

@@ -189,6 +189,20 @@ def _find_bash() -> str:
     if custom and os.path.isfile(custom):
         return custom
 
+    # Prefer our own portable MinGit install first — this way a broken or
+    # partially-uninstalled system Git can't hijack the bash lookup.  The
+    # install.ps1 installer always drops MinGit here when the user didn't
+    # already have a working system Git.
+    _local_appdata = os.environ.get("LOCALAPPDATA", "")
+    _hermes_portable_git = os.path.join(_local_appdata, "hermes", "git") if _local_appdata else ""
+    if _hermes_portable_git:
+        for candidate in (
+            os.path.join(_hermes_portable_git, "usr", "bin", "bash.exe"),  # MinGit layout
+            os.path.join(_hermes_portable_git, "bin", "bash.exe"),         # non-MinGit portable
+        ):
+            if os.path.isfile(candidate):
+                return candidate
+
     found = shutil.which("bash")
     if found:
         return found
@@ -196,7 +210,7 @@ def _find_bash() -> str:
     for candidate in (
         os.path.join(os.environ.get("ProgramFiles", r"C:\Program Files"), "Git", "bin", "bash.exe"),
         os.path.join(os.environ.get("ProgramFiles(x86)", r"C:\Program Files (x86)"), "Git", "bin", "bash.exe"),
-        os.path.join(os.environ.get("LOCALAPPDATA", ""), "Programs", "Git", "bin", "bash.exe"),
+        os.path.join(_local_appdata, "Programs", "Git", "bin", "bash.exe"),
     ):
         if candidate and os.path.isfile(candidate):
             return candidate