fix: `--force` now overwrites shared infra files during init and upgrade (#2320)

* fix: --force now overwrites shared infra files during init and upgrade

_install_shared_infra() previously skipped all existing files under
.specify/scripts/ and .specify/templates/, regardless of --force.
This meant users could never receive upstream fixes to shared scripts
or templates after initial project setup.

Changes:
- Add force parameter to _install_shared_infra(); when True, existing
  files are overwritten with the latest bundled versions
- Wire force=True through specify init --here --force and
  specify integration upgrade --force call sites
- Replace hidden logging.warning with visible console output listing
  skipped files and suggesting --force
- Fix contradictory upgrade docs that claimed --force updated shared
  infra (it didn't) and warned about overwrites (they didn't happen)
- Add 6 tests: unit tests for skip/overwrite/warning behavior, plus
  end-to-end CLI tests for both --force and non-force paths

Fixes #2319

* fix: improve skip warning to suggest specific commands

Address review feedback: the generic '--force' suggestion was
misleading when _install_shared_infra is called from integration
install/switch (which don't have a --force for shared infra).
Now points users to the specific commands that can refresh shared
infra: 'specify init --here --force' or 'specify integration
upgrade --force'.