mcpd config export: add volume contract support

[peteski22]

Is your feature request related to a problem? Please describe.

The mcpd config export command generates contract entries for environment variables and arguments, but not for volumes. There's a TODO in the codebase at internal/runtime/server.go:509:

// TODO: Export volumes similar to args and env, creating contract entries for volume paths.

This means volume paths configured via mcpd config volumes set are not exported to the environment contract file.

Describe the solution you'd like

Extend mcpd config export to generate volume contract entries similar to env vars and args:

MCPD__FILESYSTEM__WORKSPACE=/placeholder/path
MCPD__FILESYSTEM__GDRIVE=/placeholder/path

And update the portable execution context to reference these placeholders.

Additional Context

• Related to 1st class support for Docker volumes #191 (1st class support for Docker volumes)
• mcpd config export Docker volume support #196 was closed by PR Add Docker volume mount support #201 which added core volume mount support, but the export TODO remains - possibly closed prematurely before export support was implemented
• CLI volume commands (mcpd config volumes set #197, mcpd config volumes list #198, mcpd config volumes clear #199, mcpd config volumes remove #200) allow setting volumes but export doesn't handle them yet

Self-Checklist

• I have read the Contributing Guidelines.
• I have searched the existing issues and found no duplicate.
• I have provided a clear and concise description of the problem.
• I have provided a clear and concise description of the proposed solution.