ci(release): switch to v*.*.* tag trigger and add workflow_dispatch

[nekomoto911]

Summary

• Auto-trigger of Release workflow is now scoped to v*.*.* tags only. The legacy gravity-mainnet-* / gravity-testnet-* patterns are dropped — future releases follow a single v<major>.<minor>.<patch> track.
• Added a workflow_dispatch input so a release can be re-run manually from the Actions UI against any existing v*.*.* tag (useful for retrying a failed upload or pushing the docker image for a tag whose initial run was interrupted).
• Checkout step now follows the dispatched tag when invoked via workflow_dispatch; on push / pull_request it falls back to the default ref.
• Resolve tag step validates the pattern via a regex (^v[0-9]+\.[0-9]+\.[0-9]+) for both push and dispatch paths — accepts v1.7.0, v1.7.0-rc1, etc., rejects anything that doesn't start with a SemVer-shaped prefix.

Test plan

• lint-workflows (actionlint) passes on this PR.
• PR dry-run path (if: github.event_name == 'pull_request') still runs build + stage assets + docker build (no push).
• After merge to main: open Actions → Release → Run workflow → enter a known existing tag (e.g. v0.5.0) and confirm the dispatch path resolves the tag, builds, and re-uploads assets to that release with --clobber.
• Cut v1.7.0 via GitHub UI Draft Release → Publish → confirm the push path triggers and the release receives gravity_node, gravity_cli, their .sha256 files, and ghcr.io/galxe/gravity_node:v1.7.0.

Notes

• workflow_dispatch registrations are only visible on the default branch's copy of the workflow file. So the UI button will not appear until this is merged into main.
• This PR does not change the requirement that a GitHub Release must exist before the gh release upload step — that contract is unchanged.