docs: clarify hook priority validation semantics

[WOLIKIMCHENG]

Description

Clarifies the hook priority docs so they distinguish two separate paths:

• extension manifests reject invalid hook priorities during installation
• existing .specify/extensions.yml hook entries with missing or corrupted priority values are treated as priority 10 when HookExecutor.get_hooks_for_event() sorts them

Testing

• .venv/bin/python -m pytest tests/test_extensions.py::TestExtensionManifest::test_hook_priority_field_validation tests/test_extensions.py::TestHookExecutorRegistration::test_register_hooks_single_mapping_back_compat tests/test_extensions.py::TestHookExecutorRegistration::test_get_hooks_for_event_treats_missing_priority_as_default -q
• git diff --check -- docs/reference/extensions.md

[Copilot]

Pull request overview

Clarifies hook priority validation and fallback behavior.

Changes:

• Distinguishes manifest validation from stored-hook sorting.
• Documents the default priority of 10.

Show a summary per file

File	Description
docs/reference/extensions.md	Updates hook priority semantics; fallback wording needs correction.

Review details

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• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Medium

[mnriem]

Please address Copilot feedback. If not applicable, please explain why

[WOLIKIMCHENG]

Updated the hook-priority reference text to describe the current normalize_priority() behavior precisely. Stored missing, boolean, non-numeric, and non-positive values fall back to 10; numeric strings and finite floats are coerced with int(); non-finite floats are not guaranteed to fall back. This keeps the fix docs-only and aligned with the current implementation.

[Copilot]

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0 new
• Review effort level: Medium