Route Extensions contributors to Core

[sfmskywalker]

New Extensions development is consolidated in Core, while this repository still directs contributors to its own main and issue tracker. Add a prominent README notice and update CONTRIBUTING to point new work to Core, retaining source attribution and the existing 3.8/3.9 maintenance procedure. It also redirects the 41 generic create-issue links to Core while retaining numbered historical references.

Refs elsa-workflows/elsa-core#8676; part of elsa-workflows/elsa-core#8216.

Nonpublishing merge: wait for all exact-head PR checks and independent review, then use a normal merge pinned to the approved SHA with [skip ci] only in the final merge-commit subject. This follows the existing source catch-up method and suppresses the new push-triggered Packages publisher. Do not add a skip marker to the PR head or bypass required PR checks. Verify the actual merge parents/tree/subject and absence of Packages runs afterward. No workflows, support policy, publishing authority, source issues/PRs or archive settings change.

Validation: focused documentation, link-target and diff checks; ordinary PR CI and independent exact-head review are tracked below. No new runtime behavior is introduced.

[sfmskywalker]

Elsa 3 Code Review: APPROVE + HIGH @ 12ab3e0

Independent review by a separate Sol 6.1 High reviewer, relayed by the lead; the patch author did not review its own work.

Only README.md and CONTRIBUTING.md change. The new-development route points to actual Core product/contribution/register paths on main8802a883d6069d689d110008c904787b3b691042, preserving source attribution and the existing3.8/3.9 maintenance procedure. README is the exact prior content plus the notice and41 generic create-issue URL replacements; numbered historical references and statuses are unchanged. CONTRIBUTING changes are limited to the notice and scope/routing wording. Exact byte transformations, clean state and diff checks passed. No actionable findings.

This approval covers the exact documentation patch. Required CI and lead integration checks remain separate. A normal source-main merge triggers unchanged Feedz publishers; the merge hold remains until a verified nonpublishing execution method or separately authorized operational action is established. No publication, settings or archival authorization is inferred.

Retained independent report SHA-256: dd9d307840bacad3833a52e51bfde2994428d135d52caa32894a5b2756fa8ed4. Static docs/source/link-target review only; no runtime or package proof claimed.