[zumbrunn]

Closes #31.

The page editor sidebar gets one control per site taxonomy (site.taxonomies): terms as removable chips, an input to add (Enter, comma, or leaving the field) with the site's existing terms as datalist suggestions. Writes taxonomy.<name> as a list of terms. Core keys terms by the raw string, case-sensitively, so terms are not slugs: what you type is matched case-insensitively against the page's terms (no-op) and the site's existing terms (reuses that spelling), otherwise kept as typed. Pasting "a, b" adds two terms.

• Works without a blueprint; a blueprint declaring its own taxonomy field wins and the built-in control is hidden.
• Anything it does not understand (bare string, object, non-string items, taxonomy not an object) is shown as not editable and left untouched; other taxonomies' values are preserved when one is edited.
• An emptied taxonomy is removed; an empty taxonomy object is dropped (null → the PUT handler deletes the key).
• taxonomy no longer counts as an unmanaged key while the control is shown.
• GET /admin/api/pages/:path returns taxonomies (name → terms in use, most used first, capped at 500 per taxonomy).

Tests cover the pure helpers (list/absent/non-list values, preserving others, empty handling, dedupe, case-insensitive matching) and the unmanaged-keys rule. deno test passes (316).

Browser check (local site, this branch, built-in browser):

• Type a term, then click Save without Enter: this failed at first. The blur made the "Unsaved" badge appear, the toolbar re-wrapped, and Save moved out from under the click, so nothing was saved. Fixed: the badge is always laid out (hidden when clean), and save() reads the latest frontmatter from a ref. After the fix the term is in the file after Save.
• Enter and comma add a term; "qux,BAR, Zed" gave qux and Zed, and BAR was dropped because bar is already there. Removing a chip and saving rewrote the list correctly.
• A page with taxonomy: { tag: solo } shows "not a plain list", and Save left tag: solo untouched.
• Not checked: touch/mobile input, and the datalist suggestion popup.

Also fixed: the GET /admin/api/pages test context had no config.site, which made two existing tests fail on this branch.

Note: the CHANGELOG Unreleased section will conflict trivially with #32/#33; resolve by keeping all entries.

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