vscode: remove the legacy settings migration

[fi3ework]

What

Remove the settings migration (packages/vscode/src/migration.ts, the Rstack: Migrate settings command, the activation-time prompt, LEGACY_MAPPINGS, the migration tests and the README section) that maps the retired standalone extensions' rslint.* / rstest.* keys to rstack.*.

Why

• Pre-1.0 the extension owes no compatibility to earlier states, and the migration only ever served users of the two retired standalone extensions.
• The mapping already has to model dropped features (rslint.binPath / rslint.customBinPath have no rstack.* equivalent since feat(vscode): lint through an editor-shipped worker and bridge define.lint() #14) and half-surfaced edge cases (a skip-only plan is not auto-prompted — flagged in feat(vscode): lint through an editor-shipped worker and bridge define.lint() #14 review, deliberately not fixed because of this removal).
• Keeping it means every settings change in this extension carries a mapping-table update and tests for a one-off flow.

Scope

• Delete migration.ts and its tests; drop the command, the contributes.commands entry, the activation prompt and PROMPT_DISMISSED_KEY state.
• Update packages/vscode/AGENTS.md (the "settings migration exists for users of the retired standalone extensions" sentence in the pre-1.0 rule; adaptation ci: bump actions/upload-artifact to v7 for the Node 24 runtime #2 mentions the migration mapping) and the READMEs.
• Decide whether to keep a one-line README note telling standalone-extension users to re-enter their settings under rstack.*.

Refs: #14 (dropped binPath / customBinPath, added corePath).