fix(presets): treat an unreadable core template as missing

[marcelsafin]

Description

_substitute_core_template() reads the resolved core template with a bare read_text(), so one corrupted project-owned override in .specify/templates/commands/ crashes the whole wrap-strategy command registration with a raw UnicodeDecodeError. Both callers (CommandRegistrar.register_pack and _register_commands) are unguarded here — even though register_pack already skips an unreadable preset source with a warning a few lines above the call.

Fix: treat an unreadable core template like a missing one — warn and return the body unchanged with empty frontmatter — matching the function's documented no-core contract ("Both are unchanged / empty when … the core template file does not exist").

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest (6,310 passed, 176 skipped)
• New regression test test_substitute_core_template_unreadable_core_treated_as_missing (fails on main, passes with fix)
• ruff check src tests clean

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Implemented autonomously by GitHub Copilot CLI (model: Claude Fable 5) under human direction; TDD (failing test first), full suite and lint verified locally. Commit includes Assisted-by/Co-authored-by trailers.

[Copilot]

Pull request overview

Handles unreadable core templates without crashing preset command registration.

Changes:

• Warns and falls back when core templates cannot be read or decoded.
• Adds regression coverage and updates the function contract.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/specify_cli/presets/__init__.py	Handles unreadable core templates safely.
tests/test_presets.py	Tests invalid UTF-8 fallback behavior.

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Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (2)

src/specify_cli/presets/init.py:214

• CommandRegistrar has no register_pack method; the guarded caller is register_commands, while the other direct caller is PresetManager._register_skills. Referencing the nonexistent method makes this rationale difficult to verify.

    # the wrap-strategy callers already skip an unreadable preset source with
    # a warning (CommandRegistrar.register_pack).

tests/test_presets.py:10732

• This rationale names nonexistent CommandRegistrar.register_pack and says both callers skip unreadable preset sources. The actual call sites are register_commands (guarded) and _register_skills (whose source read is unguarded), so the regression explanation is inaccurate.

        The wrap-strategy callers (``CommandRegistrar.register_pack`` and
        ``_register_commands``) skip an unreadable preset source with a