feat: add custom prime template config option

[matleh]

Relates to #65

Summary

• Add templates.prime config option to .beans.yml for overriding the built-in prime document with a custom Go template file
• Custom templates have access to all built-in template variables plus {{.OriginalPrime}} (the fully-rendered built-in prime output)
• Graceful fallback: when a configured template file is missing or has parse errors, beans prime warns on stderr and outputs the built-in prime instead of failing
• beans check validates that the custom prime template file exists when configured

This allows users to add project-specific or agent-specific instructions (like those suggested in #65) without forking the built-in prime template. Users can wrap the original prime with their own additions via {{.OriginalPrime}}, or replace it entirely.

Configuration

beans:
    path: .beans
    prefix: beans-

templates:
    prime: extras/custom-prime.tmpl  # path relative to .beans.yml

Template Variables

Custom templates have access to:

• {{.GraphQLSchema}} - The GraphQL schema
• {{.Types}} - Type configs (name, color, description)
• {{.Statuses}} - Status configs
• {{.Priorities}} - Priority configs
• {{.OriginalPrime}} - The fully-rendered built-in prime output

Example

# Project-Specific Instructions

Custom guidance for this project...

{{.OriginalPrime}}

## Additional Context

More project-specific notes...

Changes

• internal/config/config.go: Add TemplatesConfig struct, Templates field on Config, ResolvePrimeTemplatePath() method
• cmd/prime.go: Extract renderPrime() function, load config, handle custom template with fallback
• cmd/check.go: Validate custom prime template file exists
• cmd/prime_test.go: New test file covering rendering, fallback, all template variables
• internal/config/config_test.go: Tests for path resolution, config loading, save/load round-trip

Design Notes

The templates section is a new top-level config key (separate from beans) to allow future extensibility:

# Future: agent-specific prime templates
templates:
    prime:
        default: extras/custom-prime.tmpl
        claude: extras/claude-specific.tmpl
        opencode: extras/opencode-specific.tmpl

[matleh]

Follow-up consideration: user-global config

We discussed adding support for a user-global config (e.g., ~/.config/beans/config.yml or $XDG_CONFIG_HOME/beans/config.yml) as a future enhancement. This would allow users to set a default custom prime template that applies across all their projects without repeating the config in every .beans.yml.

Rough shape:

• Look for a global config at a well-known path (respecting $XDG_CONFIG_HOME)
• Project .beans.yml overrides the global config (project wins)
• A global templates.prime would apply to all projects unless the project specifies its own
• Merge semantics TBD (simple override per-key? deep merge?)

This is out of scope for this PR but the templates section design accommodates it — a global config could provide default templates while projects override specific ones.