fix: include --from git+... in upgrade hint to avoid PyPI squat package (#2411)

The compatibility-error messages in extensions.py and presets.py, plus the
extension troubleshooting guide, told users to upgrade with:

    uv tool install specify-cli --force

Without `--from git+https://github.com/github/spec-kit.git`, uv resolves
`specify-cli` from PyPI, where an unrelated package with the same name
(no author, no project URLs) ships a stub CLI that lacks `extension`,
`preset`, and most spec-kit commands. Users following the upgrade hint
land on the squat package and report "extension command removed"
(see #1982).

Reuse the existing `REINSTALL_COMMAND` constant in extensions.py and
import it from presets.py so all three call sites point at the GitHub
source. The doc fix also adds a one-line note explaining the PyPI
collision so the same advice doesn't get re-stripped later.

Refs #1982

src/specify_cli/extensions.py

@@ -1108,7 +1108,7 @@ class ExtensionManager:
                 raise CompatibilityError(
                     f"Extension requires spec-kit {required}, "
                     f"but {speckit_version} is installed.\n"
-                    f"Upgrade spec-kit with: uv tool install specify-cli --force"
+                    f"Upgrade spec-kit with: {REINSTALL_COMMAND}"
                 )
         except InvalidSpecifier:
             raise CompatibilityError(f"Invalid version specifier: {required}")

src/specify_cli/presets.py

@@ -27,7 +27,7 @@ import yaml
 from packaging import version as pkg_version
 from packaging.specifiers import SpecifierSet, InvalidSpecifier
 
-from .extensions import ExtensionRegistry, normalize_priority
+from .extensions import REINSTALL_COMMAND, ExtensionRegistry, normalize_priority
 
 
 def _substitute_core_template(
@@ -576,7 +576,7 @@ class PresetManager:
                 raise PresetCompatibilityError(
                     f"Preset requires spec-kit {required}, "
                     f"but {speckit_version} is installed.\n"
-                    f"Upgrade spec-kit with: uv tool install specify-cli --force"
+                    f"Upgrade spec-kit with: {REINSTALL_COMMAND}"
                 )
         except InvalidSpecifier:
             raise PresetCompatibilityError(