feat(presets): Composition strategies (prepend, append, wrap) for templates, commands, and scripts

[Copilot]

Presets can now augment lower-priority templates instead of only fully replacing them. A new optional strategy field in preset.yml controls how content is composed across the priority stack.

provides:
  templates:
    - type: "template"
      name: "spec-template"
      file: "templates/spec-addendum.md"
      strategy: "append"  # new field; default: "replace"

Strategy	Behavior	Templates	Commands	Scripts
replace	Full replacement (existing default)	✓	✓	✓
prepend	Insert before lower-priority content	✓	✓	—
append	Insert after lower-priority content	✓	✓	—
wrap	{CORE_TEMPLATE} / $CORE_SCRIPT placeholder substitution	✓	✓	✓

Multiple composing presets chain recursively (e.g. security prepend + compliance append → header + core + footer).

Python resolver (src/specify_cli/presets.py)

• VALID_PRESET_STRATEGIES / VALID_SCRIPT_STRATEGIES constants; validation in _validate() rejects prepend/append for scripts
• PresetResolver._collect_all_layers() — walks full priority stack, reads strategy from each preset manifest
• PresetResolver.resolve_content() — bottom-up composition across layers
• PresetManager._register_commands() — composes command content before writing to agent directories

Shell resolvers

• resolve_template_content() in scripts/bash/common.sh
• Resolve-TemplateContent in scripts/powershell/common.ps1

CLI

• specify preset resolve shows composition chain when non-replace strategies are present

Documentation

• presets/scaffold/preset.yml — strategy field docs and composition examples
• presets/README.md — composition strategies section; moved implemented items out of future considerations
• presets/ARCHITECTURE.md — strategy table and content resolution function references

Tests

• 26 new tests: strategy validation (valid/invalid/script restrictions), resolve_content() for each strategy × type, multi-preset chaining, override precedence, separator behavior, _collect_all_layers() ordering