chore: Replace manual publish with changesets

[Winify]

• Add @changesets/cli for per-package versioning
• Replace workflow_dispatch release.yml with changesets/action (Version Packages PR on push to main → publish on merge)
• releaseType input is replaced by changeset files: pnpm changeset picks which packages + what bump
• Only packages with pending changesets get published

What

Why

How

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Architecture self-check

Required for every non-trivial PR. If a box is unchecked, explain why.

• No new duplication. This PR does not add a type, constant, enum, or contract that already exists in another package. (If it consolidates one, note which item from CLAUDE.md §7 is being resolved.)
• No cross-adapter imports. No code in service, nightwatch-devtools, or selenium-devtools imports from another adapter.
• No adapter imports in backend / app. Neither package reaches into adapter internals.
• Typed contracts at boundaries. Any new fetch(...), ws.send(...), or HTTP route has a typed request/response shape in shared (or in service types if shared doesn't exist yet, with a TODO to move).
• No if (framework === '...') outside an adapter. Framework branching uses a typed FrameworkId.
• No new any at package boundaries. Internal any is acceptable only at a documented framework-edge with a one-line comment.

Multi-adapter changes

• This PR touches more than one adapter package.

If checked: why isn't this in core? Answer here:

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Debt scoreboard

List the CLAUDE.md §7 debt items this PR resolves, partially resolves, or extends. Delete this section only if the PR genuinely affects no debt items.

• Resolved: <item, or "none">
• Partially resolved: <item, or "none">
• New debt introduced: <item, or "none — and explain why if any>

If new debt is introduced, it must be added to CLAUDE.md §7 in this PR.

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Testing

• Unit tests for new logic in shared / core (required per CLAUDE.md §4).
• Regression test for any bug fix (required per CLAUDE.md §4).
• pnpm build passes.
• pnpm test passes.
• pnpm lint passes.
• For UI/runtime changes: verified in example/ (or example for the framework I changed).

If any required item is skipped, say so here with the reason:

<your note, or "n/a">

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Screenshots / recordings (UI changes only)