feat: OACP v0.4.0

[kiloloop-release]

OACP v0.4.0.

Highlights (full details in the CHANGELOG 0.4.0 section):

• Optional message signing: Ed25519 detached-JWS auth trailer over raw message bytes (sender opt-in via the crypto extra), receiver verify-before-parse with warn-mode annotations, receiver-local trust pins with an integrity-checked import/revoke flow, and a byte-exact signing conformance corpus with tamper coverage.
• Canonical trailer encodings: exactly one wire spelling per signed message; encoding aliases fail verification.
• Autonomy instrumentation: pinned four-value completion_kind enum, co-occurring reason codes, checkpoint pause provenance (paused_at_utc, breach_basis), evaluator self-stamped provenance, and human-outcome recording for checkpoint-paused records.
• Granular task-profile capabilities (merges_pr, files_issues) compiled into the runtime envelope, with the repo-targeting gate hardened across flags, URL positionals, and environment.
• oacp trust revoke, oacp init --oacp-dir, doctor trust-root drift advisories, and envelope completion fixes (in-session clear, bookkeeping exempt from the file budget).

Quality gate: full preflight (lint, shellcheck, tests) green at the promoted head; wheel + sdist build verified; fresh-venv install smoke with the crypto extra passed.

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