agda.yml installs an unpinned `apt-get install -y agda` at L51 and L176 — the prover under the proofs changes without a commit

[hyperpolymath]

Measured (2026-09-22, main = 9c4b72b)

• .github/workflows/agda.yml:51 and :176 sudo apt-get install -y agda git (no version) on ubuntu-latest, with the stdlib cloned at v2.3.
• Positive control: epistemic-types' Proof Safety runs in a digest-pinned debian:13-slim container with agda-bin=2.6.4.3-1+b2 agda-stdlib=2.1-4, asserts agda --version, and passes --double-check; residual-evidence-types adopted the same recipe on 2026-09-22 (run 35773288367 green) with zero third-party actions.
• Three stdlib versions are in play across the family (2.1 in residual, 2.3 here, 2.1-4 distro in epistemic-types).

Why it matters

An image bump changes the prover under the proofs without a commit: the next ubuntu-latest rollover changes the apt agda version and no receipt records which one checked the suite.

Acceptance criteria

1. Both steps install an explicit version (agda-bin=<ver> in a digest-pinned container, or an apt pin with a checksum), and agda --version is asserted equal to the pinned string before any module is checked.
2. Watched-failing → green: grep -c 'apt-get install -y agda git' .github/workflows/agda.yml is 2 today, 0 after.
3. The chosen stdlib version is recorded in PROOF-STATUS with the measured warning count under it (measure the UnsupportedIndexedMatch warning count under each candidate stdlib before choosing; do not assume).
4. Mutant: change the pinned version string to one that does not exist; the job fails at install, not silently later.

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