Surface repo-bootstrap commands as a tree node, not just Command Palette

[VeryComplexAndLongName]

Summary

• repo-bootstrap-snippets shipped three real commands (Generate Agent Instructions, Configure Dependabot, Generate Path-Scoped Copilot Instructions) but made them Command Palette-only by design decision — no tree/menu presence.
• Live-usage report: not discoverable without knowing the exact command name to search for — the same class of gap already fixed twice this session for other features.
• Adds a "Repository Setup" node to the Changes tree (right after "OpenSpec Configuration"), always visible. Its three leaf items invoke the existing commands unchanged, including their project-type QuickPick prompts — no new commands, no duplicated logic.
• Archive tree is untouched — the node only makes sense in Changes.
• Stacked on Archive three shipped OpenSpec changes #42/Withhold Delete Task for done tasks, even in active Changes #43 (already-merged prerequisite branches this was cut from).

Test plan

• npm run verify (typecheck/lint/english-policy/test) passes repo-wide
• New/updated unit tests in changes-tree.test.ts: root includes the node right after config; expanding it returns the three commands with correct command.command ids and contextValue
• Real Extension Host run (npm run test:integration) — 6/6 passing
• openspec change validate --strict repo-bootstrap-tree-ui passes

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