feat: add app versioning

[liamnwhite1]

Summary

Add app-level executable version discovery so Myna applications can inspect external executable versions and branch on compatibility or template behavior.

Related issues

• Closes feat: allow versioning of apps #182

Details

This feature adds shared executable version helpers to MynaApp:

• get_executable() resolves the configured --exec value with a default fallback.
• get_executable_version() runs a configurable version command, captures stdout and stderr, supports --env setup and Docker execution, and extracts a version identifier with either a regex or the first non-empty output line.
• extract_executable_version() centralizes output parsing for app-specific version logic.

The developer guide now documents how app authors can call the new helper after argument parsing and use the returned version string to select templates or raise compatibility errors.

Unit tests cover regex extraction, default first-line extraction, stderr output from non-zero commands, unmatched regex failures, and environment-file PATH setup.

Impact

This is a low-risk feature addition. Existing executable validation behavior is preserved, with the duplicated executable fallback logic moved into get_executable().

The change gives app developers a common extension point for executable compatibility checks without requiring each app to implement its own subprocess/version parsing logic. No existing app templates or execution workflows are changed by default.

Testing strategy

Local environment:

• OS/shell: Linux, bash
• Python: 3.12.10 from the project .venv
• Install method: existing project virtual environment

Commands run:

• .venv/bin/python -m pytest
• .venv/bin/ruff check src/myna/core/app/base.py tests/test_app_versioning.py
• git diff --check

The default pytest suite passed locally. The new tests use temporary fake shell executables, so no external simulation application versions are required for coverage.

Checklist

• Architecture/docs impact considered. Updated ARCHITECTURE.md, developer docs,
  and/or ADRs where needed.