Make spacetime describe support human-readable versions

[krisajenkins]

Run spacetime describe mydb today and it refuses: --json is a required
flag, and its help text promises that "in the future, omitting this will give
human-readable output". This patch brings that glorious future. 😁

spacetime describe [db] now prints the module as readable text, in the
sections Tables, Views, Reducers, Procedures, and Types:

You can narrow it to every entity of one kind, or to one entity by name. The
entity types are tables, reducers, procedures and types, matching the
section headings:

    $ spacetime describe wikiwatch procedures
    fetch_edits(timer: FetchEditsSchedule)  [private]
    fetch_previews(timer: FetchPreviewsSchedule)  [private]

    $ spacetime describe wikiwatch types Thumbnail
    Thumbnail = { url: String, width: U32, height: U32 }

The singular forms (table person) still work, since that's what the
JSON-only command accepted. --format json, or the existing --json
shorthand, prints the raw definitions instead: the whole module, an array for
a listing, or a single def. Text is styled like the migration plan that
spacetime publish prints, and is coloured only when stdout is a terminal and
NO_COLOR is unset.

The renderer is a new describe module in the schema crate, next to the
migration printer whose look it shares.

• StyledWriter: the colour scheme, colour/no-colour buffer and indent
  helpers move out of TermColorFormatter into a pub(crate) writer that
  both printers use. Migration output is unchanged, and its snapshots pass
  untouched.
• type_name spells types language-neutrally: U64, Array<T>,
  Option<T>, Timestamp, and named types by their scoped name joined with
  .. It reads the "for generate" typespace, which keeps special types and
  refs intact.
• Field and variant names come from the case-converted typespace, because
  typespace_for_generate keeps source names (imageUrl, not image_url).
  Column defaults are formatted through WithTypespace, so a sum prints as
  (red = ()) rather than ( = ()).
• Reducer names arrive already qualified with their submodule
  (lib.end_session), while table, view, procedure and type names are local
  and have the prefix added. Prefixing reducers too would print
  lib.lib.end_session, and a test pins that.
• Types lists only named types reachable from a column or a signature, and
  leaves out table row types, which their table's block already shows.
  types <name> searches every named type, so a row type seen in a signature
  (FetchEditsSchedule) can still be looked up.
• Each listing (describe_tables and friends) shares its sorted source with
  the whole-module renderer, and tests pin each one to its module section.
• CLI: --format text|json defaults to text, using a Format enum now
  shared with sql and logs in common_args. --json conflicts with an
  explicit --format. Whole-module JSON is still the raw, unvalidated def,
  returned before validation, so a module that fails validation can still be
  dumped for debugging. Existing JSON output is byte-for-byte unchanged.
• Tests: insta snapshots of a fixture covering every section, submodules,
  indexes, constraints, defaults and schedules. The describe smoketest now
  checks the text output, exact single-entity output, --json against
  --format json, and the flag conflict.
• Docs: the regenerated CLI reference, the cheat sheet, and the CLI agent
  skill, plus its codex-plugin copy, which must match byte for byte.
• Known gap: inside a submodule table, a column whose type is defined in that
  submodule prints the bare type name, while Types prefixes it with lib..

API and ABI breaking changes

None. spacetime describe <db> without --json used to be an error, so no
existing invocation changes meaning, and JSON output is byte-for-byte
unchanged. --json combined with an explicit --format is now rejected as a
conflict, but --format is new to this command.

Rollback safety impact

n/a

Expected complexity level and risk

2. The diff is large, but most of it is the new, self-contained renderer and
   its snapshots. The parts that touch existing code are the StyledWriter
   extraction from the migration printer (its snapshots pass untouched) and
   the shared Format enum now used by sql and logs.

Testing

• Insta snapshots for the renderer: whole module (colour and no colour), a single table, a single reducer.
• Unit tests pinning each listing to its module section, and submodule reducer names not being double-prefixed.
• describe smoketest covers text output, exact single-entity output, --json vs --format json, and the flag conflict.
• Reviewer: run spacetime describe against a real module of your own and check the output reads well.