feat(templates): add py: lines to command templates' scripts frontmatter (#3403)

* feat(templates): add py: lines to command templates' scripts frontmatter

Every templates/commands/*.md with a scripts: block now declares a py:
variant so --script py renders a Python invocation via the existing
interpreter-prefixing in process_template.

Fixes #3283

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: install scripts/python for the py script type

install_shared_infra mapped every non-sh script type to powershell, so
--script py rendered invocations pointing at files that were never
installed. Map py to the python variant dir and skip __pycache__
artifacts during the copy.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* test: read templates with explicit utf-8 encoding

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: keep py: lines standalone, enforce script existence in tests

Drop the plan/tasks py: lines that referenced scripts shipping in the
core port (#3280); they move to that PR. Tests now assert every py:
line points at a script the repo ships, so a dangling reference can
never merge green.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/specify_cli/__init__.py

@@ -141,8 +141,9 @@ def _install_shared_infra(
 
     Copies ``.specify/scripts/<variant>/`` and ``.specify/templates/`` from
     the bundled core_pack or source checkout, where ``<variant>`` is
-    ``bash`` when *script_type* is ``"sh"`` and ``powershell`` when it is
-    ``"ps"``.  Tracks all installed files in ``speckit.manifest.json``.
+    ``bash`` when *script_type* is ``"sh"``, ``python`` when it is ``"py"``,
+    and ``powershell`` when it is ``"ps"``.  Tracks all installed files in
+    ``speckit.manifest.json``.
 
     Shared scripts and page templates are processed to resolve
     ``__SPECKIT_COMMAND_<NAME>__`` placeholders using *invoke_separator*

src/specify_cli/shared_infra.py

@@ -403,7 +403,7 @@ def install_shared_infra(
     # manifest entries the core no longer ships (stale-script cleanup, #3076).
     seen_rels: set[str] = set()
     scripts_scanned = False
-    variant_dir = "bash" if script_type == "sh" else "powershell"
+    variant_dir = {"sh": "bash", "py": "python"}.get(script_type, "powershell")
 
     def _decide_overwrite(rel: str, dst: Path) -> tuple[bool, str | None]:
         """Return (write, bucket) where bucket is 'skip', 'preserved', or None."""
@@ -465,6 +465,10 @@ def install_shared_infra(
                     for src_path in variant_src.rglob("*"):
                         if not src_path.is_file():
                             continue
+                        # Python bytecode caches are local artifacts, not
+                        # workflow scripts — never install them.
+                        if "__pycache__" in src_path.parts:
+                            continue
                         # Mark scanned only once a real source file is seen. An
                         # empty (or symlink-skipped) variant keeps this False, so
                         # stale-cleanup is skipped — otherwise it would treat every