fix: address second round of review comments

1. Log warning on auto-install failure instead of silent catch
2. Fix README install command (--from → --dev)
3. Bash: fail fast if resolve_template/json_escape unavailable after git-common.sh fallback
4. PowerShell: fail fast if Resolve-Template unavailable after git-common.ps1 fallback
5. Fix PowerShell $env:FEATURE_DIR → $FEATURE_DIR in specify.md
6. Fix docstring to reflect already-installed return value

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>
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