feat(cli): add q2 get-config to print merged document config as JSON

[cscheid]

Summary

Adds q2 get-config, a CLI command that returns a document's effective configuration after Quarto 2's full metadata-merge semantics — project _quarto.yml, directory _metadata.yml layers, document front matter, and format.<fmt>.* flattening — as JSON. External tools (editors, linters, build wrappers) can read resolved config without reimplementing any of Quarto's resolution rules.

Closes #256.

q2 get-config <file> [yaml-path] [--to <fmt>] [--output value|pandoc] [--strict] [--compact]

• No path → entire merged metadata; a dot-separated path (e.g. format.html.toc, authors.0.name) selects a value; a numeric segment indexes an array.
• Missing path → null, exit 0 (or --strict to error).
• --to picks the format whose format.<fmt>.* block is flattened in (default html).
• --output value renders prose (e.g. title) as a Markdown string; --output pandoc emits a self-contained, source-free Pandoc AST fragment.

How it works

The merged metadata is the ast.meta produced at the document-profile checkpoint, so the command runs only [ParseDocumentStage, MetadataMergeStage] — the same MetadataMergeStage a real render uses (one merge code path, no engines/filters/rendering). This keeps get-config consistent with rendering and cheap.

Examples

$ q2 get-config report.qmd title
"Hello *world*!"

$ q2 get-config report.qmd title --output pandoc
[{"t":"Str","c":"Hello"},{"t":"Space"},{"t":"Emph","c":[{"t":"Str","c":"world"}]},{"t":"Str","c":"!"}]

$ q2 get-config report.qmd toc --to html   # true
$ q2 get-config report.qmd toc --to pdf    # false
$ q2 get-config report.qmd authors.1.name  # "Bob"

Changes

• pampa::config_json — ConfigValue → clean JSON projection (ProseMode::{Value, Pandoc}) + dot-path navigation with array indices.
• pampa::writers — qmd::write_inlines; json::{inlines,blocks}_to_source_free_json (reuse the maintained writers, strip s/l/attrS).
• quarto-core::get_config::merge_document_metadata — runs the parse+merge prefix and returns ast.meta + ASTContext; re-exports the projection helpers so the CLI stays within the quarto-core API.
• quarto get-config CLI subcommand.
• Docs — docs/guide/get-config.qmd.

Tests

• 12 unit (pampa), 4 integration (quarto-core), 11 CLI e2e driving the real q2 binary (quarto).
• Full workspace cargo nextest run green (9542 passed); cargo xtask verify green including the WASM/hub-client build (pampa feeds the WASM image).

Open question for review

Default --to is html, matching render's actual default (render only supports native/html today and bails otherwise). Implementing "first declared format" now would make get-config inconsistent with render; deferred until render honors it. See the plan (claude-notes/plans/2026-06-02-get-config-command.md) for the full design and decisions D1–D8.

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