Upgrade the Anthropic SDK, OpenAI SDK and commander majors deliberately #47

Upgrade the major-version dependencies deliberately, with the code changes each needs, instead of by unattended Dependabot PRs. Dependabot opened these, and they are closed in favour of this issue:

• @anthropic-ai/sdk 0.37 → 0.128 (chore(deps): bump @anthropic-ai/sdk from 0.37.0 to 0.128.0 #41). It builds and passes the tests as is, including the Anthropic streaming test. Check thinking/effort for current models (newer Claude models use adaptive thinking and an effort setting instead of a fixed budget_tokens) and prompt caching (A real system prompt and prompt caching #12).
• openai 4 → 7 (chore(deps): bump openai from 4.104.0 to 7.23.0 #44). tsc fails: ChatCompletionMessageToolCall is now a union that includes custom tool calls, which have no .function (src/backends/openai.ts). Narrow to function calls; check reasoning_effort and streaming usage.
• commander 12 → 15 (chore(deps): bump commander from 12.1.0 to 15.0.0 #42). Check that allowUnknownOption, variadic [prompt...] and option parsing still behave the same (covered by test/invocation.test.mjs).

Each upgrade is its own PR with a version bump. .github/dependabot.yml now groups minor and patch updates and ignores majors, so these come back only when we choose to take them.