Refactor ci.skeleton.yaml to eliminate template duplication

[tsavo-at-pieces]

Overview

File: `templates/github/workflows/ci.skeleton.yaml`

The skeleton template has two nearly identical blocks: `single_platform` (lines 41-149) and `multi_platform` (lines 151-307). The checkout, git config, dart setup, cache, protobuf, and analysis steps are copy-pasted between them. The only structural difference is that multi-platform splits `analyze` and `test` into separate jobs with a matrix.

Risk

Any bug fix or improvement to a shared step must be made in two places. History shows these drift — it's exactly the kind of duplication that causes "fixed on ubuntu but not on windows" bugs.

Examples of recent or potential drift:

• Cache key format (currently identical, but if one is updated and not the other...)
• Action versions (checkout, setup-dart, cache)
• Git config organization list
• Token handling

Potential Approaches

1. Mustache partials — Extract shared step sequences into partial templates
2. Programmatic YAML builder — Build the YAML in Dart code, composing shared steps
3. Single template with conditional job splitting — More complex Mustache logic but one source of truth

Acceptance Criteria

• Shared steps defined in exactly one place
• Both single-platform and multi-platform paths still produce correct output
• Existing tests (once written per issue #XX) still pass
• Consumer workflows regenerate identically

[tsavo-at-pieces]

PR #23 (merged) added sync-marker comments to all duplicated step sequences in ci.skeleton.yaml and fixed the analysis cache key drift (runner.arch was missing in the multi-platform block).

The structural duplication between single-platform and multi-platform blocks remains. A full refactor (e.g. extracting shared steps into a reusable composite action or Mustache partial) is still open work.

[tsavo-at-pieces]

Progress update (PR #23 merged):

Added # ── shared:*section* ── keep in sync with {single,multi}_platform ── sync-marker comments throughout the skeleton template. This makes it explicit which blocks must stay in sync between the two sections and helps catch drift during review.

The structural deduplication (extracting shared steps into a reusable composite action or YAML anchor) remains open for a future pass. The sync markers are a pragmatic intermediate step.