docs: document YAML configuration

[coderberry]

What?

Add a canonical YAML configuration guide for both the global config.yaml file and project-specific .heard.yaml files.

The guide documents current-main settings, defaults, scope, precedence, project labels, narration preferences, direct-YAML event filters, provider and privacy controls, credential safety, inspection commands, and troubleshooting. It is linked from the README's Tuning and self-host sections, and the contributor checklist now requires configuration documentation to stay in sync.

Why?

Heard's configuration surface has grown faster than its documentation. Users could discover .heard.yaml, but not reliably determine which options worked globally, which were project-capable, how nested project files resolved, or which persisted keys were internal or inert.

How?

The reference is derived from the current config loader, CLI validation, preference schema, adapter behavior, daemon consumers, and tests. It separates supported controls from internal state, calls out the nearest-file and shallow merge rules, documents adapter-specific timing limitations, and warns against placing credentials in repository files.

Testing?

• .venv/bin/ruff check heard/ tests/ — passed.
• .venv/bin/pytest -q — 956 passed in 570.77 seconds.
• Parsed all six YAML examples as mappings.
• Reconciled 37 documented config keys, four direct-runtime keys, and all ten
  preference slots against current source.
• git diff --check upstream/main..HEAD — passed.

Anything Else?

This is documentation-only and changes no runtime behavior. Review should focus on the scope and precedence descriptions in docs/configuration.md.