Replace executable manifests with a hardened data-only command contract

[hyperpolymath]

Outcome

Replace executable Elixir manifest loading with a strict, versioned, capability-limited check manifest and hardened command gateway.

Scope

• Define a data-only manifest schema for check IDs, argv, absolute target workdir, capabilities, artifacts, timeouts, resource limits, GitHub contexts, and trust policy.
• Remove Code.eval_file from untrusted/consumer manifest ingestion.
• Canonicalise and validate workdirs against configured checkout roots.
• Validate argv length, environment allow-lists, artifact paths, check IDs, and freeze destinations.
• Add per-check timeouts, process-tree termination, output limits, and explicit mutation policy.
• Fuzz manifest/envelope decoders and test symlink, traversal, injection, and oversized-input cases.

Acceptance criteria

• Consumer manifests cannot execute code during parsing.
• Invalid paths, unknown capabilities, malformed argv, and excess resource declarations fail before execution.
• Command execution remains argv-based with no implicit shell.
• The existing dogfood manifest and at least two external-repository fixtures round-trip through the new schema.