build: raise minimum Go version to 1.25

[777genius]

Raises the declared build minimum to Go 1.25.0 for the upcoming managed-filesystem and MCP SDK integrations. Updates the three OS minimum CI lanes and contributor documentation; GOTOOLCHAIN=local prevents those jobs silently switching to a newer toolchain. The Go 1.26 lanes remain.

Stacked on #153. This prerequisite belongs before the runtime installer PR because it uses Go 1.25 filesystem APIs. No runtime behavior or dependency versions change here; SDK integration remains separate.

Validation: diff/manifest review passed. Direct qualification downloads for Go 1.25.14 and the exact 1.25.0 minimum returned HTTP 404 from the official archive endpoint in the hosted environment. No successful local minimum-toolchain run is claimed; this draft's minimum CI lanes must pass before readiness. Existing Go 1.26 results do not replace that requirement.

[coderabbitai]

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