[User Story] Add templatefile processing in instance module main.tofu

[noahwhite]

Story Summary

As a platform engineer, I want to add templatefile processing for compose.yml and env.config in the instance module, so that configuration values are interpolated into the templates before being passed to Ignition.

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✅ Acceptance Criteria

• locals block updated to process compose.yml.tftpl with templatefile function
• locals block updated to process env.config.tftpl with templatefile function
• Static files (Caddyfile, snippets, mysql-init) loaded via filebase64
• All template variables passed to ghost.bu via ct_config data source
• Base64 encoding applied to all file contents for Ignition

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📝 Additional Context

• Design: Templates use ${var} syntax for OpenTofu interpolation
• Static files don't need templatefile - use filebase64 directly
• The ct_config data source already exists - just add new template variables

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📦 Definition of Ready

• Acceptance criteria defined
• Template files created (blocked by story 1)
• Variables defined (blocked by story 4)
• Story is estimated
• Team has necessary skills and access
• Priority is clear
• Business value understood

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✅ Definition of Done

• All acceptance criteria met
• OpenTofu syntax valid (tofu validate passes)
• Peer-reviewed
• Template interpolation working correctly

[linear]

GHO-50