Summary

Add a detect-changes composite action that maps changed paths to named filters, so monorepo consumers can run only the CI jobs affected by a change. This is the monorepo enabler referenced by #168 §10 and is the most relevant of the dynamic-matrix pair (this + #29).

Scope decision (still relevant — yes): turbo-themes is a real multi-package monorepo consumer, so path-based change detection has a concrete adopter. Prefer evaluating a SHA-pinned dorny/paths-filter wrapped behind our action contract over hand-rolling git-diff logic; only build bespoke if the third-party action can't meet our egress/pinning policy.

Problem

• No built-in support for detecting which packages/services changed in a monorepo.
• Consumers must hand-write git-diff scripts or pull in an unpinned third-party action.
• feat(workflows): standardize reusable workflows as drop-in CI for all lgtm-hq repos #168 §10 (monorepo path detection) has no implementing component.

Proposed Solution

.github/actions/detect-changes (wrapper around a pinned dorny/paths-filter, or bespoke fallback):

- uses: ./.github/actions/detect-changes
  id: changes
  with:
    filters: |
      frontend:
        - 'packages/frontend/**'
        - 'packages/shared/**'
      backend:
        - 'packages/backend/**'
      docs:
        - 'docs/**'
        - '*.md'

- name: Test frontend
  if: steps.changes.outputs.frontend == 'true'
  run: bun run test --filter=frontend

Inputs

inputs:
  filters:   { description: 'YAML mapping of names to path patterns', required: true }
  base:      { description: 'Base ref (default: auto-detect)', required: false }
  head:      { description: 'Head ref', default: 'HEAD' }
  token:     { description: 'GitHub token', default: '\${{ github.token }}' }

Outputs

outputs:
  changes:  { description: 'JSON object of all filter results' }
  # plus one boolean output per filter (e.g. frontend: 'true')

Implementation Notes

• Prefer wrapping dorny/paths-filter pinned to a full SHA; expose our standard input contract and harden-runner/egress defaults around it.
• Detection modes: PR (base..head), push (before..after), manual (explicit base).
• If a dynamic matrix is needed downstream, emit a JSON output consumable by feat(actions): add generate-matrix action for dynamic workflows #29 — feat(actions): add generate-matrix action for dynamic workflows #29 is only required if/when that pathway lands.
• Add BATS coverage for filter parsing and boolean output behavior.

Benefits

• Unlocks efficient monorepo CI for turbo-themes and future multi-package repos.
• Implements the missing feat(workflows): standardize reusable workflows as drop-in CI for all lgtm-hq repos #168 §10 component.
• Pinned, policy-compliant wrapper instead of ad-hoc per-repo scripts.

References

• feat(workflows): standardize reusable workflows as drop-in CI for all lgtm-hq repos #168 §10 — monorepo path detection
• feat(actions): add generate-matrix action for dynamic workflows #29 — generate-matrix (downstream consumer of change outputs; dependent)
• dorny/paths-filter — candidate upstream to wrap