v0.60.0

The editor learns to copy-edit. Two features land together on the markdown source: a spellcheck that
runs as you write, and an opt-in tidy that reads a draft once with a language model and proposes a
light copy-edit you review before any of it lands.

Spellcheck is on by default. Misspelled words pick up a quiet amber underline, and the correction
popover offers ranked suggestions, an add-to-dictionary action, and an ignore-for-this-session
action, all keyboard-reachable. It runs locally on a Web Worker, so no text leaves the browser, and
it reads the markdown structure: code, links, frontmatter, layout-block machinery, and media:
tokens are never flagged. A second quiet layer catches the objective slips spellcheck misses: a
doubled word, a double space inside a line, a stray run of punctuation. The dialect is declared once
per site under spellcheck.dialect (default en-us), so a British site loads the British word list
and "colour" reads as correct. The personal dictionary is a git-committed file at
src/content/.cairn/dictionary.txt, so a word one editor adds is shared with the rest through the
same commit pipeline the content uses.

Tidy is opt-in and off until a developer enables it. When on, an editor runs it over the whole
document or a selection, and cairn reads the draft once through the Anthropic API and computes the
diff locally. The review is a step-in diff dialog: insertions show in green, deletions struck through
in red, and the author's original stays in the buffer until they apply. Objective fixes come pre-kept;
a judgment edit (a configured style normalization, a grammar reword) carries a review-this treatment
and a plain-language reason, and it is not swept by Accept fixes until confirmed. The prompt is built
from the site's own convention config and never harmonizes to the author's habits or guesses an
undeclared style, so an author's voice is preserved. Output is validated as a proofread, not a
restructure: a result that changes the heading structure, the frontmatter, a media: token, a code
block, or more than a bounded fraction of the wording is discarded with an honest message and the
document is left untouched. Conventions are edited in a two-tier settings screen and stored in the
committed site config under tidy.conventions.

New dependencies: @codemirror/lint (the surfacing layer for both spellcheck and the objective-error
underlines), @anthropic-ai/sdk (the Worker-side tidy model call, guarded off the client), and
spellchecker-wasm plus its bundled English dictionary asset (the spellcheck engine, delivered from
the packaged dist so the Worker and the word list reach a consumer build).

No consumer action is required for an existing site. Both features are additive. Spellcheck replaces
the browser's native spell checking with cairn's own, so an upgrading editor sees the new amber
underline and the in-editor correction popover in place of the browser's right-click menu, with no
config change needed. Tidy gives a site nothing until a developer turns it on: set tidy.enabled: true
in the site config, add the ANTHROPIC_API_KEY Worker secret, and optionally pick a model and
conventions. cairn doctor checks that the key is configured once tidy is enabled. The editor
walkthrough is in write in the editor, the developer setup is in
enable tidy and the editor copy-edit, and the design rationale is in
the editor copy-edit.