Make templates fully harness-agnostic: model aliases, deprecate image/auth

[ptone]

Summary

Templates should be strictly harness-agnostic — they define the agent's role, not its execution mechanics. This PR:

• Adds model size aliases (small, medium, large) to harness-configs, resolved to concrete model names at provision time. Templates can use model: large instead of model: opus, making them portable across Claude, Gemini, and other backends.
• Deprecates harness-specific fields (image, auth_selectedType, concrete model names) in scion-agent.yaml with warnings, guiding users to move these to harness-config config.yaml.
• Updates documentation (templates.md) to clarify the template/harness-config boundary and document model aliases.

Closes ptone#98. Implements the model alias portion of #42.

Changes

• pkg/config/settings_v1.go: Add ModelAliases to HarnessConfigEntry
• pkg/config/templates.go: Add ResolveModelAlias(), WarnDeprecatedTemplateFields(), KnownModelAliases
• pkg/agent/provision.go: Wire alias resolution and deprecation warnings into provisioning flow
• pkg/config/schemas/agent-v1.schema.json: Mark image, auth_selectedType as deprecated; update model description
• pkg/config/schemas/settings-v1.schema.json: Add model_aliases property to harness config schema
• pkg/hubclient/types.go, pkg/store/models.go: Add ModelAliases to HarnessConfigData
• pkg/harness/*/embeds/config.yaml: Seed default model aliases for all harnesses
• docs-site/src/content/docs/advanced-local/templates.md: Document model aliases, clarify harness-agnostic boundary

Backward Compatibility

• Concrete model names in templates pass through unchanged
• image and auth_selectedType still work as override values
• No breaking changes to API types or persisted config

Test plan

• Unit tests for ResolveModelAlias() — alias resolution, concrete passthrough, nil/empty handling
• Unit tests for WarnDeprecatedTemplateFields() — all deprecated field combinations
• Unit tests for KnownModelAliases — canonical alias set validation
• Existing template and merge tests pass
• go vet and go build pass

[gemini-code-assist]

Code Review

This pull request makes templates harness-agnostic by deprecating harness-specific fields (such as image, auth_selectedType, and concrete model names) in the agent configuration and introducing abstract model size aliases (small, medium, large). These aliases are mapped to concrete models within the harness configuration and resolved during agent provisioning. The feedback suggests adding a defensive nil check to WarnDeprecatedTemplateFields to prevent potential nil-pointer dereference panics.

[gemini-code-assist]

To prevent potential nil-pointer dereference panics, add a defensive nil check at the beginning of WarnDeprecatedTemplateFields.

Suggested change

	func WarnDeprecatedTemplateFields(cfg *api.ScionConfig) []string {
	var warnings []string
	func WarnDeprecatedTemplateFields(cfg *api.ScionConfig) []string {
	if cfg == nil {
	return nil
	}
	var warnings []string