#1181 gh shim: every Plexus and relay refusal explains what happened and whether a retry is safe

Every refusal the GitHub shim relays from Plexus or AFT's own relay now prints the operator-approved explanation: what happened and whether a retry is safe.

• Explanations: each Plexus refusal_code and relay refusal maps to one approved line. All 72 table rows are pinned as independent literal strings, including the reach, actor and rerun suffixes.
• Bypass hint: a not_governed refusal in a head shell adds the approved operator-bypass line. Worker shells never get a bypass hint, and their refusal lines stay exact.
• Warming: module_warming retries once after 5 s with the same request ID, so the write goes out at most once. After that it refuses with the approved "nothing reached GitHub" text.
• Plexus's own message: where Plexus sends detail.message (for example review_thread_not_own), it's shown through the vendored contract vectors and the real-shim wire test.
• Unchanged: the fail-closed manifest check (#1120), the audited operator bypass (#1112), bot-speech binding selection (#1115) and -F @file reading (#1129). The branch is merged with current main.

Tests: gh_shim_relay_client_tests, gh_shim_speech/policy_tests, gh_shim_runtime_context_test and gh_shim_wire_goldens_test. Eight AFT-owned stderr exchange snapshots are updated to the approved wording. Plexus request and response bytes are unchanged.

The worker's final runs on Linux 12-core passed the gh_shim library tests, including both Landlock canaries, the wire goldens and the Windows compile check. My re-run on the merged branch was refused by a full build server, so this PR's CI is the gate.

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Summary by cubic

Every refusal the GitHub shim relays from Plexus or AFT's own relay now prints an operator-approved explanation: what happened and whether retrying is safe.

• Each Plexus refusal_code maps to one approved literal line, including reach, actor, and rerun suffixes; all 72 table rows are pinned as independent strings.
• module_warming refusals retry once after 5 s with the same request ID so the write goes out at most once, then refuse with the approved "nothing reached GitHub" text.
• not_governed refusals in a head shell add the audited operator-bypass hint (GH_SHIM_BYPASS=operator); worker shells never get it.
• Plexus detail messages (such as review_thread_not_own) are preserved verbatim.
• Existing behavior is unchanged: the fail-closed manifest check, audited operator bypass, bot-speech binding selection, and -F @file reading. Plexus request and response bytes are unchanged.

Tests

• gh_shim_relay_client_tests, gh_shim_speech/policy_tests, gh_shim_runtime_context_test, and gh_shim_wire_goldens_test cover the new wording; eight AFT-owned stderr snapshots are updated.