Executor as MCP client: speak spec 2026-07-28 outbound

[RhysSullivan]

Executor as an MCP client on spec 2026-07-28 (part 1 of 2; the server half is #1611).

Moves packages/plugins/mcp from @modelcontextprotocol/sdk v1 to @modelcontextprotocol/client/core 2.0.0, plus the three stale ^1.12.1 manifest floors aligned to ^1.29.0.

• Streamable HTTP connections negotiate the era automatically (versionNegotiation: auto); forced-SSE and stdio keep the SDK legacy default.
• The endpoint shape probe advertises 2025-11-25 and adds a server/discover fallback for 2026-07-28-only servers.
• Transport errors classify via v2 types (SdkHttpError.status, SseError.code, InsufficientScopeError) instead of matching SDK-internal message strings.
• Elicitation and tools/list_changed handlers use v2 method-string registration.
• src/testing/ stays on SDK v1 deliberately as a legacy-wire interop fixture.

Fixes the reported failure: executor sent 2025-11-25 to a modern-only endpoint and was rejected (Unsupported protocol version). Verified against a legacy: "reject" server — the old client reproduces the report byte-for-byte; this client negotiates, lists tools, and round-trips callTool.

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commit: a0201c9

[RhysSullivan]

Folded into #1611 (single consolidated PR per review preference).