up: add details for setting context via arg or config file

[gememma]

mirrord PR: metalbear-co/mirrord#4679

• Added a Context section under Configuration with a table showing precedence for various methods of setting context
• Added context under common config fields
• Added services.*.context config field
• Added --context under CLI args
• Removed the leading $ from shell code blocks in this file so that they are copy and paste-able

[linear-code]

COR-1770

[greptile-apps]

Greptile Summary

Adds documentation for selecting Kubernetes contexts in mirrord up, including CLI and configuration-file options.

• Documents precedence among --context, services.*.context, common.context, and the current context.
• Adds the new context fields and CLI argument to the configuration reference.
• Removes shell prompt markers so commands can be copied directly.

Important Files Changed

Filename	Overview
docs/using-mirrord/multiple-concurrent-sessions.md	The new context example, precedence table, configuration fields, and CLI documentation agree with the corresponding mirrord implementation and tests.

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