refactor: drop retired backward-compat code

[hashiiiii]

Summary

Remove three pieces of code kept only to preserve compatibility with retired internal implementations: the prerelease MCP protocol version 2024-10-07, and the extension's push-less synchronous diff pipeline.

Motivation

Closes #59.

PrefabLens is pre-1.0, so no deprecation cycle is required. The 2024-10-07 MCP version was accepted only because the (now-removed) TypeScript SDK/host echoed it; it is not a released MCP spec version. The extension's if (!push) "Compatibility path" is a second, fully synchronous pipeline that no production caller uses — every semanticDiff request originates in a tab content script, which always supplies push.

Changes

• cli/src/mcp.zig: drop 2024-10-07 from supported_versions and its comment. A client that sends it now negotiates to the default 2025-06-18 (already covered by the "falls back to the default version for unknown ones" test). Remove the 2024-10-07-echo regression test.
• extension/src/background/handler.ts: make push required on Handler.semanticDiff, delete the push-less compatibility branch (and the then-dead repoKey local), and update the resolveRemaining comment that referenced it. The two-stage push pipeline is now the only path.
• extension/src/background/index.ts: supply push unconditionally via a thin adapter that no-ops for a (non-existent) non-tab sender, instead of passing undefined.
• extension/src/background/handler.test.ts: migrate the suite to the push API. A new resolveFully helper drives a request to its final push and returns the fully-resolved response, so the resolution assertions read the pipeline's end state unchanged.

Testing

• zig build lint → clean; zig build test → pass; zig build perf → 50000 objects in 140 ms (ceiling 600 ms).
• extension: tsc --noEmit → clean, biome ci . → exit 0, vitest run → 142 passed (13 files), npm run build → ok.