feat: implement preset wrap strategy

[kennedy-whytech]

Description

Implements strategy: wrap for preset commands, which was listed as a roadmap
item in presets/README.md. When a preset command sets strategy: wrap, the
{CORE_TEMPLATE} placeholder in its body is substituted with the body of the
installed core speckit command template at install time.

This lets preset authors add pre/post logic around core commands without
copy-pasting the entire command body — useful for extending frequently-updated
templates like /speckit.specify or /speckit.tasks. Personally, I find it neccessary.

Changes

• src/specify_cli/presets.py — adds _substitute_core_template() helper;
  wires substitution into _register_skills (skills-backed agents like Claude)
• src/specify_cli/agents.py — wires substitution into register_commands
  (all other agent types)
• presets/self-test/ — adds a speckit.wrap-test command for E2E coverage
• presets/README.md — marks wrap as implemented for command/template types

Test results

184 tests pass (tests/test_presets.py), including a new TestWrapStrategy
class with unit tests for the helper and an end-to-end install test.

Manual test results

Agent: Claude Code | OS/Shell: macOS/zsh

Command tested	Notes
specify preset install with strategy: wrap command	{CORE_TEMPLATE} correctly substituted in written skill/command file

AI assistance disclosure

This PR was implemented with AI assistance (Claude Code) for code generation,
test writing, and commit messages. All changes were reviewed and verified
manually.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

AI Disclosure

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