context: assemble DB→opus blob + skald show-context CLI

skald-core::context is the bridge between 'rows in postgres' and
'prompt-ready markdown blob.' ContinuationContext::assemble(pool,
parent_story_id, recent_n) pulls:

  - parent story meta (title, series, total word count)
  - characters split real / fictional
  - canon_facts grouped by category
  - chapter summaries for everything older than the recent window
  - FULL prose for the last recent_n chapters

render_markdown() formats it with the most-condensed data first
(characters, canon) and the richest detail last (recent chapter
prose). Opus reads it linearly so by the time it's writing the new
chapter, the previous chapter's prose is freshest in its context
window.

The 'continuation reads ≥85% of parent' rule lands here via
parent_coverage() which counts recent prose + summaries-as-proxy
(250 words / summary) against parent word_count. The web UI / CLI
can warn before firing a gen pass if coverage is below threshold.

New CLI subcommand:

  skald show-context --story <uuid> --recent <N>

Assembles + prints the blob to stdout (eprintln'd stats summary
goes to stderr). No LLM call — pre-flight inspection so we see
what would be sent before paying for it. Useful for prompt-eng
work in the next session.

Module structure now:

  skald-core/
    config.rs     ForgeConfig
    context.rs    ContinuationContext (new)
    db.rs         connect_and_migrate
    forge.rs      Forge — three-pass orchestration
    ingest.rs     markdown parser
    models.rs     row types
    lib.rs        MIGRATOR + module exports

  skald/
    main.rs       clap CLI
    serve.rs      axum + /health + migrations
    import.rs     skald import-markdown
    show_context.rs  skald show-context  (new)

skald-core/src/lib.rs

@@ -5,6 +5,7 @@
 //! this crate knows about any specific story. Every story is rows.
 
 pub mod config;
+pub mod context;
 pub mod db;
 pub mod forge;
 pub mod ingest;