--stdin on every model-reading verb, and delete the mgtt surface

[sajonaro]

Two things, in the order they have to happen: add the general capability, then remove the special case.

--stdin — additive, nothing breaks

Every model-reading verb gains --stdin. It is a general convenience for any generated model, not a concession to one bridge: a model that arrives on a pipe never has to touch the disk.

--stdin is stripped before positionals are counted, which is what makes it decidable. query, derive and compare all take further positionals after the model, so dropping it silently would shift the rest left; with the flag removed, the remaining arity is fixed and the sentinel goes into the model slot the verb already has. No verb, and nothing in the engine, learns that stdin exists.

Two shapes needed a decision:

• compare OLD.writ --stdin — --stdin fills the new side and must follow the old model, or compare --stdin baseline.writ and compare baseline.writ --stdin would strip to the same one positional and one of them would silently compare backwards.
• query --stdin NAME --claims FILE — query finds its questions by convention (MODEL.writ → MODEL.claims), and a piped model has no beside; it resolved <stdin>.claims and reported a file nobody named. --claims overrides the sibling and is required with --stdin, which now says so. The convention stays the default, so every existing invocation is unchanged, and this matches check, which has had the flag all along.

docs/tour.md listed "there is no --claims flag on it" under things that will catch you once — the worst place for a stale rule. Corrected there, in the cheat sheet, and in the README table.

Deleting the mgtt surface

The translation now lives in mgt-tool/mgtt2writ.

writ is a language. SQL is a notation standard that does not move; mgtt is a product with a schema that does. One such reading is defensible — a second would make writ a collection of bridges wearing a language's name, and deferring that to a trigger nobody would notice was the wrong call.

Removed: tooling/mgtt/ (1643 lines), tooling/cli/cmd_mgtt.ml, the dispatch arm, the help entry, tests/unit/test_mgtt.ml and its fixture, docs/mgtt-bridge.md (moved to mgtt2writ as docs/reading.md), and the README section. 3167 deletions, 4 insertions.

Nothing in writ names mgtt now.

Verification

make test green — 26 test binaries, including test_stdin: 14 checks. make lint clean apart from four files with pre-existing drift (cmd_sql.ml, mcp/bin/*, sql_ast.ml) that I deliberately left alone.

The pipeline it enables, verified end to end against the real binaries:

$ mgtt model export --json | mgtt2writ | writ check --stdin
states: 6   edges: 9    …    exit 0

Rationale: ADR 0001.