docs(templates): add variable vs coder_parameter guidance, deprecated fields, registry PR conventions

[bpmct]

Adds three sections to the templates skill based on patterns that come up when building real templates against a homelab Coder deployment.

Changes

variable vs coder_parameter — clarifies when each should be used. Infrastructure-level choices fixed per deployment (host arch, storage pool, remote host) belong in variable and are set by the admin at push time. Per-workspace choices (image, cpu, memory) belong in coder_parameter. Includes a concrete example and the push command with --variable.

Deprecated coder_agent fields — dir is deprecated in recent provider versions, generates warnings on every coder templates push, and breaks Coder Desktop file sync. Worth calling out explicitly since it appears in older templates that get copied as a starting point.

Contributing templates to coder/registry — links to AGENTS.md and the PR template rather than duplicating them, and adds three non-obvious gotchas: no .tftest.hcl required for templates, the PR title convention (feat(<namespace>/templates/<name>): ...), and syncing the fork's main with upstream before branching to avoid a noisy diff.

Type of Change

• New skill
• Enhancement to existing skill
• Bug fix
• Documentation only

[DevelopmentCats]

LGTM