refactor(workflows): consolidate pd-store-server publish flow (#16)

* refactor(workflows): consolidate pd-store-server publish flow

- add dedicated reusable workflow for latest/release pd-store-server publishing
- split multi-arch publishing into amd64 first, arm64 second, then manifest merge
- introduce latest schedule and hash gate using LAST_SERVER_HASH
- remove standalone server latest/release workflows and update README map

* fix(workflows): stabilize cache export and reuse mvn layers

- switch cache export to mode=min with ignore-error to avoid hard failures
- add shared plus module cache scopes for better cross-module Maven layer reuse
- separate amd64 and arm64 cache scopes to reduce cross-arch cache contention
- add upstream refactor notes for Dockerfile and artifact-based build dedup

* chore(workflows): apply review clarity fixes
* fix(workflows): harden strict mode and smoke flow
* fix(workflows): tighten cache writes and readme wording
* fix(workflows): switch pd-store-server cache to module-only

Author: imbajin