Add configurable default install target (apm config target) with install-time fallback precedence

[Copilot]

Description

apm install required repeating --target for common workflows. This change adds a persisted default target and wires it into target resolution without changing existing override semantics.

• Config key surface

  • Added target to apm config:
    • apm config set target <value>
    • apm config get target
    • apm config unset target
  • Added config helpers in apm_cli.config:
    • get_install_target()
    • set_install_target()
    • unset_install_target()

• Install resolution behavior

  • install/phases/targets.py now resolves target precedence as:
    1. CLI --target
    2. apm.yml target / targets
    3. persisted apm config target
    4. filesystem auto-detect
  • Explicit --target and manifest targets continue to override the config default.

• Tests and docs alignment

  • Added focused unit coverage for config storage/CLI flows and target-phase precedence.
  • Updated CLI/reference docs and apm-guide command corpus to include the new key and precedence chain.

apm config set target claude
apm install                  # uses claude when --target and apm.yml target(s) are absent
apm install --target cursor  # explicit flag still wins

Type of change

• Bug fix
• New feature
• Documentation
• Maintenance / refactor

Testing

• Tested locally
• All existing tests pass
• Added tests for new functionality (if applicable)

Spec conformance (OpenAPM v0.1)

If this PR changes behaviour that an OpenAPM v0.1 req-XXX covers,
confirm the three-step ritual (see CONTRIBUTING.md "Adding or
changing a normative requirement"):

• Spec edit: docs/src/content/docs/specs/openapm-v0.1.md updated
  (new/changed <a id="req-XXX"></a> anchor + prose + Appendix C
  row).
• Manifest edit: docs/src/content/docs/specs/manifests/openapm-v0.1.requirements.yml
  updated.
• Test edit: a @pytest.mark.req("req-XXX") test under
  tests/spec_conformance/ added or extended.
• CONFORMANCE.{md,json} regenerated via
  uv run --extra dev python -m tests.spec_conformance.gen_statement
  and committed.
• N/A -- this PR does not change OpenAPM-observable behaviour.