[Phase B1] Introduce Persona and OutputFormat domain concepts (registry + strategy)

[terisuke]

Tracked under ADR 0002 — Phase B. Detail: implementation plan §B1.

Problem

The current draft pipeline assumes one author and one output format. Hard-coded examples:

• internal/domain/article/draft.go:25-35 requires a # title first line and rejects code fences.
• internal/application/draft/prompt.go:12-51 injects "Noteにそのまま貼り付けられる日本語Markdown記事" into every system message.
• internal/infrastructure/note/fetcher.go:234-246 rejects any host other than note.com.

To support both Terisuke (本人) and Cloudia (架空キャラクター) on note / cor-jp blog / Zenn / Qiita / homepage, we need two orthogonal first-class concepts in the domain.

Scope

New packages:

• internal/domain/persona
  • types: Persona, PersonaID, PersonaSeed, Registry
• internal/domain/format
  • types: OutputFormat, FormatID, Validator, Registry

Hooks:

• domain/article.NewDraft accepts format_id, dispatches validation to the format's Validator instead of the current hard-coded rules.
• domain/brief.ArticleBriefSession gains persona_id, output_format_id.
• application/draft.BuildPrompt becomes BuildPrompt(persona, format, guide, brief) and pulls a per-format system fragment.

Acceptance criteria

• go test ./... passes with at least the existing note_article format registered.
• A unit test exists per format validator (added in Phase B3).
• No conditional if format == "note_article" exists in application/handler code; all branching is through strategy dispatch.
• BuildPrompt is unit-tested for each registered format.
• Existing scenario tests (cor_instrument × note) produce byte-identical or near-identical drafts to before this refactor (regression safety).

Out of scope

• Specific format implementations beyond note_article (those land in Phase B3).
• Persistence of persona/format ids (Phase C).
• Source fetcher changes (Phase B2).

Dependencies

• Should land before B3 (templates) and B4 (persona seeds).

[terisuke]

Implemented in PR #33 against develop. The PR adds Persona and OutputFormat registries, mode-aware draft prompting, format-aware validation, API selectors, and embedded format guides. Remaining Phase B work is tracked separately in #22, #23, #24, and #25.