feat(stovepipe): add queueconfig Store with default implementation

[mnoah1]

What?

Introduce queueconfig.Store and wire process to resolve per-queue max_concurrent for use when checking if we are ready to admit another build.

Why?

MVP logic to enqueue builds has two constants that will need to become configurable per queue in the future:

1. concurrent builds per queue (currently defaulting to 1)
2. delay when we're not ready to build the latest head and need to re-enqueue it as a future process task to re-check

Test Plan

• Unit tests below

[behinddwalls]

Clean extension slice — the Store interface mirrors the SubmitQueue queueconfig precedent exactly, config stores are the sanctioned exception to the extension input contract, no factory impls or routing in the extension package, and the README follows the prose-only rule. Two small inline notes, nothing blocking.

[behinddwalls]

⚠️ Automatic stack rebase failed

This PR could not be automatically rebased after its base PR was merged. The rebase hit conflicts that need manual resolution.

To fix manually:

git fetch origin
git checkout mnoah1/stovepipe-process-queueconfig
git rebase --onto origin/main 2b2eb79ca6029db0ce819d1825d2b6dc37edf62f mnoah1/stovepipe-process-queueconfig
# resolve conflicts, then:
git push --force-with-lease

Then update this PR's base branch:

gh pr edit 334 --base main