Support HCL as an alternative agent config format

[dgageot]

Adds HCL as an alternative authoring format for agent configuration files,
alongside YAML. HCL documents are converted to YAML at load time, so the
existing schema, version dispatch, and migration chain are reused unchanged.

What it looks like

agent "root" {
  description = "An agent that talks like a pirate"
  instruction = "Always answer by talking like a pirate."
  model       = "auto"

  welcome_message = <<-EOT
  Ahoy! I be yer pirate guide, ready to set sail on the seas o' knowledge!

  What be yer quest? 🏴‍☠️
  EOT
}

Conventions:

• Top-level keyed maps (agents, models, providers, mcps, rag) are
  written as labeled blocks: agent "root" { … } ↔ agents.root: { … }.
• Inside an agent, command "name" { … } ↔ commands.name: { … }.
• Toolsets use the label as the type field:
  toolset "mcp" { … } ↔ toolsets: [{ type: mcp, … }].
• Multi-line strings use heredocs. Because HCL templates expand \${…},
  any literal \${…} in instructions must be escaped as \$\${…}.

See examples/pirate.hcl and examples/gopher.hcl — both produce
configurations byte-identical to their .yaml siblings (locked in by
TestHCLExamplesMatchYAML).

How it's wired

HCL bytes ──hcl.ToYAML──► YAML bytes ──read version──► v0…v8 parser ──migrate──► latest.Config

The HCL → YAML step happens once, before anyone looks at the version field.
Detection is by .hcl extension, with a content-sniff fallback (LooksLikeHCL)
for OCI artifacts and other sources that don't have a filename hint.

The converter is version-agnostic: it applies stable structural rules
(labeled-block → keyed map, etc.) that target the broad shape every schema
version has shared since v0. Field-level renames between versions are still
absorbed by the existing up-migrators. The blockRules table is the union of
HCL conventions across all supported versions; new entries are additive.

Drift protection

• TestHCLExamplesMatchYAML — every .hcl example produces a config equal
  to its .yaml sibling.
• TestHCLBlockRulesCoverSchemaMaps — walks agent-schema.json and asserts
  every top-level property of shape {additionalProperties: …} has a matching
  modeMapByLabel rule registered. Verified to fail with a clear message
  when a new keyed map is added to the schema without an HCL rule.
• TestJsonSchemaWorksForExamples — now also validates .hcl examples
  (via the converter) against the JSON schema.

Notable design choices

• Singular labeled blocks (agent "root", model "claude") rather than
  agents { root { … } }. Idiomatic HCL (cf. Terraform resource "type" "name") and lighter to write.
• Toolset label = type so multiple toolset "mcp" { … } blocks aggregate
  into the YAML toolsets: list with type injected.
• yaml.MapSlice for label-keyed outputs so agent declaration order
  is preserved through the YAML-marshal step (Agents is a slice in the
  Go config; order matters).
• No HCL function/variable evaluation. Expressions are evaluated with an
  empty hcl.EvalContext{}, so templates can't reach out to anything.

Files

File	What
pkg/config/hcl/hcl.go	HCL → YAML converter
pkg/config/hcl/hcl_test.go	Unit tests for the converter
pkg/config/config.go	Detect HCL in Load, convert before YAML pipeline
pkg/config/resolve.go, pkg/sandbox/args.go	Recognize .hcl extension
pkg/config/examples_test.go	Cover .hcl examples + sibling-equivalence test
pkg/config/schema_test.go	Schema validation for .hcl + drift test
examples/pirate.hcl, examples/gopher.hcl	Drop-in HCL versions of existing examples

Validation

mise lint and mise test are both green. ./bin/docker-agent agent run --dry-run
loads both example HCL files successfully.

[rumpl]

This is cool

[docker-agent]

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