probe: discover environments from manifests, not a hardcoded list

The environment list was written out four times — probe.py's shim tuple and its
results dict, tests/test_probe.py's ENVIRONMENTS, and ci.yml's wheel loop — so
adding an environment meant four edits and five workstreams collided on three
files. Discovery is now a tomllib scan of environments/*/pyproject.toml plus a
@probes(name) registry, with an optional [tool.arena] tasksets key for one
package that ships several (tera_spatial declares four).

An environment discovered without a registered probe WARNS rather than exits 1,
because interactive environments land their engine before their probe. That is
temporary; it becomes a hard failure with the last registration.

The warning is not a licence to stop being gated. test_probe.py pins the
ungated set to an explicit PENDING_PROBES allowlist that only ever shrinks —
without it, removing a @probes decorator moved the name from the table into a
warning line that already carries five, and every remaining assertion stayed
vacuously true while the environment silently stopped being gated.

Also untracks three environments/*/uv.lock files, which had been failing the
lock-policy step on main.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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