feat(cli): add hydrate stage discard

[rennehan]

Abandoning a staged changeset meant deleting .hydrate/stage.json by hand. clear is not that verb — it stages removal of every top-level node, which is nearly the opposite and a dangerous thing to reach for by mistake.

$ hydrate stage discard
+ boundary Disc1
    description: authored prose that exists nowhere else
+ behavior Disc1.Disc2 (out: x:T)
Discarded 2 staged operations on branch 'sweep-1': 2 nodes, 0 edges.
Recoverable from .hydrate/stage.discarded.json until the next discard.

$ hydrate stage discard
Nothing staged on branch 'sweep-1'; nothing to discard.        # exit 0

Why it prints the operations

The discarded work exists nowhere else — nothing was committed, so there is no server copy. A node's description is the user's authored prompt for code generation, and 2 nodes does not tell you what you lost. The listing goes through the renderer diff already uses, so what scrolls past is a real record.

Why there is no prompt

This CLI is driven non-interactively and a prompt would break piping. So the mitigation is recoverability, not friction: the outgoing stage is parked in .hydrate/stage.discarded.json (single slot, overwritten each time), and --json echoes the deltas so an agent can re-stage without reading the file. The park happens before the delete, and a park failure aborts it.

Blast radius

Exactly one file. .hydrate/ also holds the binding and a 100KB+ pulled index, so a wider reach would unbind the working copy or throw away a snapshot the user would have to re-fetch. write_state_file takes a file name, not a path, so a caller cannot escape the state directory. Pinned by a test that plants a binding, an index, and a sibling file and asserts all three survive.

A test that didn't test anything

The unit test proving park() works did not prove discard() calls it — deleting the park call left every unit test passing, because the test invoked park directly. The end-to-end test that runs the real binary in a real working copy catches it; verified by re-applying the mutation.

status's noun renderer is now shared rather than copied, so the three verbs describing one stage can't drift on what it contains.

415 unit + 9 integration tests. fmt, clippy -D warnings clean. Driven against a real bound project, both the populated and empty paths.