DEVEX-1628: optional skip_ci on php-common-bump

[pdodgen-revparts]

Adds an optional skip_ci input (default true = current behavior) so nightly common bumps can omit [skip actions] and trigger a build/tag. Backward-compatible: no existing caller changes behavior. Nightly workflows opt in via separate PRs.

Refs DEVEX-1628

[cursor]

PR Summary

Medium Risk
Changes shared CI commit behavior used by many repos; default true limits blast radius, but skip_ci: false will trigger push pipelines and integration deploys when nightly workflows opt in.

Overview
Adds an optional skip_ci input (default true) to the shared php-common-bump reusable workflow so bump commits only append [skip actions] when callers want to suppress CI. The commit step now reads skip_ci and commit_message via env and builds the message suffix conditionally instead of always skipping workflows.

This unblocks DEVEX-1628: nightly common-update jobs can opt in with skip_ci: false in follow-up PRs so revolutionparts/common bumps on main trigger build/tag (and integration deploy where applicable). Existing callers (merge-path upgrade-common, daemons, batch, etc.) keep today’s behavior without changes.

Also adds design and implementation docs under docs/superpowers/ describing the nightly opt-in rollout across 11 service repos.

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