Licence — a three-way contradiction, now settled

This repo declared three different licences at once. Its own
docs/testing-and-bench-strategy.adoc already diagnosed it and said the fix needed an
owner decision:

"Licence single-source: guix/pixi/stapeln say PMPL-1.0-or-later, 255 headers say
MPL-2.0, only LICENSES/MPL-2.0.txt exists — a 3-way contradiction ... any FIX is
owner-manual, file-by-file"

Owner ruling applied: MPL-2.0 for code, CC-BY-SA-4.0 for documentation.

The most visible symptom: README.adoc declared PMPL-1.0-or-later as the primary
licence and linked LICENSES/PMPL-1.0-or-later.txt — a file that does not exist. A
broken licence link on a public README, contradicting both LICENSE and GitHub's own
MPL-2.0 classification.

PMPL is not a registered SPDX identifier, so those manifest fields were invalid to
any tool that read them, not merely inconsistent.

Historical/diagnostic mentions of PMPL (CHANGELOG, tech-debt notes, the strategy doc
quoted above) are deliberately left alone — they describe the problem rather than
declare a licence.

GitHub already classified this repo MPL-2.0, so no change was needed there; this makes
the tree agree with it.

guix.scm — a real parse hazard

Line 2 was // Owner: ... — a C-style comment in a Scheme file. // is not a
comment in Guile, so guix shell -f guix.scm could not have worked. Now ;;.

Dead code and a fake gate

closes #265 — deletes the orphaned OCaml "Choreographic Sessions" prototype:
src/lexer.mll, src/parser.mli, src/parser.mly, test/test_typecheck.ml, and the
stray test_minimal.ml in the repo root. Verified fully orphaned: no dune file, no opam
file, no CI step, nothing in the tree imports them.

closes #266 — deletes .github/workflows/boj-build.yml. To answer the question in
the issue directly: it is not an implementation of the cartridge, it is a trigger
that POSTs to boj-server asking it to build this repo. It could never have worked, and
was reporting green anyway:

• http:// — plaintext, violating estate policy
• boj-server.local — an mDNS hostname, unresolvable from GitHub-hosted runners
• malformed JSON payload (stray escaping plus an extra closing brace)
• continue-on-error: true — which masked all three as a passing check

That combination is a textbook fake gate: it has been showing "BoJ Server Build Trigger
success" on the board while doing nothing.

Follow-up, not done here

With the OCaml prototype gone, the ocaml 5.4.1 pin in .tool-versions is vestigial.
Left in place deliberately — that file is the single source of truth for the prover
toolchain and isn't worth churning in this PR.

Verification

• git grep PMPL -- '*.toml' '*.scm' '*.a2ml' returns nothing.
• LICENSES/ now contains exactly MPL-2.0.txt and CC-BY-SA-4.0.txt, matching what
  the tree actually declares — reuse lint was previously guaranteed to fail on the
  missing PMPL file.
• Governance's "Licence consistency" job should stay green; the workflow-security linter
  loses one file to check.

🤖 Generated with Claude Code