Add project-template delete + three new built-in templates

[VeryComplexAndLongName]

Summary

• deleteProjectTemplate() in packages/core, a new POST /api/templates/delete REST endpoint, and a VS Code command openspec-ui.deleteProjectTemplate scoped to viewItem == openspec-ui.projectTemplate only. Built-in templates never show a delete action anywhere — they're code, not workspace data.
• Fixes a real gap found while dogfooding "Customize": there was no way to remove a customized project-level template short of manually deleting the directory.
• Adds three built-in templates: flask-to-fastapi (Python), a language-agnostic flat-to-hexagonal-architecture, and node-vitest-testing-baseline (Node.js/TypeScript) — balances the catalog against the original Python-only seed and covers the Node.js/TypeScript gap directly.
• Extends the archived template-catalog capability spec (openspec/specs/template-catalog/spec.md) with an ADDED requirement for deletion.

Test plan

• npm run verify green across all five workspaces (372 tests total), re-run after git add per this repo's English-policy-check gotcha.
• openspec change validate --strict template-catalog-v2 passes.
• Real, live smoke test (not just unit tests): built and ran the actual standalone server against this repository, drove the Templates tab in a real browser, and deleted the actual leftover project template from earlier manual testing — confirmed gone on disk afterward. Also confirmed the confirmation gate genuinely blocks an undismissed/declined confirm (didn't delete until the dialog was accepted), and that all 4 built-in templates never show a delete action. VS Code side has no live-UI verification available in this environment (no desktop-automation tool) — relies on 27 passing commands.test.ts cases plus both hosts sharing the identical core function verified live. Full detail in openspec/changes/template-catalog-v2/smoke-test-notes.md.

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