Licence: add a proof-of-usage condition for AI systems (CC-BY-4.0 unchanged)

[ebellumat]

This adds a proof-of-usage condition to the licence, and the files that make it
work. The existing CC-BY-4.0 text is kept byte-for-byte and the condition is appended below it, so no existing term changes.

The condition: an automated system that trains on, indexes, embeds or derives output
from this repository records that it did, and credits the repository where it already
publishes credits — model card, about page, acknowledgements. No fee, no revenue share,
no restriction on what anyone builds with the code.

What is in the diff

File	Why
LICENSE-DOCS.md	The condition itself.
AGENTS.md	The file an automated system opens first, so the condition is stated where it will actually be read.
PROOF_OF_USAGE.md	The register the condition refers to. Records are appended here.
proof-of-usage.json	The same thing machine-readable, so npx proof-of-usage record finds where records go instead of guessing.
README section	The human-facing explanation, at the bottom, under its own heading.
package.json	license becomes SEE LICENSE IN LICENSE-DOCS.md. The terms now differ from plain CC-BY-4.0, and leaving the old SPDX id there would mislead every tool that reads the field.

Records are taken as pull requests against main — this repository's default
branch. The format is Proof of Usage PoU/1.0,
which is specified independently of any licence.

Worth knowing before you decide

Adding an obligation to a permissive licence means the result is source-available, not
OSI open source — Origami says so plainly in the licence's own README, and it is the
real cost of the condition. Some organisations refuse source-available dependencies and
some registries flag them. If that trade is not worth it here, the same request works
beside your existing licence as a plain NOTICE.md, which changes nothing legally and
keeps the project open source — npx proof-of-usage init writes that variant instead.
Happy to convert this PR to that shape if you prefer it.

Disclosure: I am affiliated with Origami, which authored MIT-PoU. I am proposing it
here because I use this project, not on anyone's behalf.

Relicensing needs the agreement of the copyright holders and any contributors whose work
is still present — that call is entirely yours, and closing this is a perfectly fine
answer.

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