feature(config): add writing-voice override to .han/config.md

[mxriverlynn]

Summary

Adds a new writing-voice frontmatter key to the .han/config.md project config, letting a consuming project point Han's readability surfaces at its own writing-voice profile instead of the built-in Han voice.

Behavior

• Absent or blank: the built-in profile at han-communication/references/writing-voice.md applies, with no mention of the config.
• Set to a file that exists (path relative to the working directory): that file replaces the built-in profile wholesale for the run, including the vocabulary blocklist the readability rule enforces.
• Set to a file that does not exist: the skill warns the user and asks whether to use the built-in Han voice or skip the writing voice entirely for the run. Skipping applies the readability rule with no voice profile and no blocklist. This is an explicit, documented exception to the config rule's silent note-and-degrade behavior.

Changes

• han-core/references/config-rule.md (canonical): defines the writing-voice key and names the ask-instead-of-note exception in the degradation section. All vendored copies re-synced byte-identical.
• han-communication:readability-guidance: Step 1 now resolves the voice source (configured file, warn-and-ask fallback, or built-in) before reading the standard, so every prose-producing consumer picks the override up automatically.
• han-communication:edit-for-readability: resolves the voice in Step 1 and carries the outcome (file path, skip instruction, or nothing) into the readability-editor dispatch.
• readability-editor agent: accepts a named voice-profile file in place of the built-in profile, or a skip instruction that drops only blocklist enforcement.
• Docs: docs/configuration.md annotated example and new writing-voice section (overview made count-free), plus matching updates to the long-form docs for both skills and the agent.