fix(integration): refresh shared infra on `integration switch` (#2375)

* fix(integration): refresh shared infra on integration switch

* fix(integration): address Copilot review on switch shared-infra refresh

- Clarify install_shared_infra docstring: force overwrites regular files
  but always preserves symlinks (safe-destination check refuses to follow).
- Print refresh_hint only for preserved_user_files; skipped_files keeps
  the generic remediation. Avoids misleading guidance when files were
  merely skipped (not detected as customized).
- Catch ValueError from the safe-destination check and bucket the path
  under a new symlinked_files warning instead of aborting the switch.
- Restore templates/constitution-template.md to upstream (drop accidental
  leading blank lines).

* fix(integration): narrow symlink bucketing to dedicated exception

Address Copilot feedback on shared_infra.py:305 — _safe_dest_or_bucket
caught any ValueError as 'symlinked', which masked genuine safety errors
(path escape, parent-not-a-directory).

- Introduce SymlinkedSharedPathError(ValueError) raised only by the
  symlink-specific branches in _ensure_safe_shared_*().
- _safe_dest_or_bucket() now catches only SymlinkedSharedPathError;
  other ValueErrors propagate so the operation aborts with the real
  cause instead of being silently bucketed.
- Wrap top-level dest_scripts/dest_variant/dest_templates mkdir calls
  in the same bucket helper so a symlinked .specify/scripts or
  .specify/templates is preserved with a warning rather than aborting
  the switch (matches the documented 'preserve customizations' behavior).
- Update tests to expect the new bucket+warn behavior for leaf-level
  symlinked destinations.

* fix(integration): tailor shared-infra warnings and rename preflight test

Address Copilot review on PR #2375:

- skipped_files hint now uses refresh_hint when refresh_managed=True
  so integration switch suggests --refresh-shared-infra instead of the
  generic init/upgrade flags.
- symlinked-files warning header says "path(s)" rather than "file(s)"
  since symlinked directories (e.g. .specify/scripts/bash) are also
  bucketed there.
- Rename test_shared_infra_install_preflights_before_writing to
  test_shared_infra_install_buckets_unsafe_destinations_and_continues
  to match the new bucket-and-continue semantics.

* test: rename symlink bucketing tests to reflect bucket-and-continue behavior

The two file-bucketing tests at line 300/320 were named *_refuses_*, but
the new behavior buckets symlinked file destinations with a warning while
safe destinations in the same install still complete. Rename to
*_buckets_* and update docstrings to match.

The remaining *_refuses_* tests (line 342/362/381) genuinely raise on
symlinked dirs/manifests and keep their names.

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Co-authored-by: Quratulain-bilal <quratulain.bilal@users.noreply.github.com>