fix(#829): guard prefixed runtime variable launches

docs/scratchpads/829-mutator-gate.md

@@ -121,3 +121,10 @@ Carried authority chain also verified/read for the locked T-B gate contract: SPE
 - Both independent gates converged on one guard-only completeness gap at round-4 head `1eb77c17f3147d4fa9944f77f1826243135b9cc0`; all round-4 primitive anchoring, command-position parsing, 14/14 inventory, broker ordering, and coverage remain locked-good.
 - Reproduced exactly: a temporary production source containing `launcher=claude` followed by `exec "$launcher" -p x` exits 0 with `0 gated / 0 total`. The literal command resolver skips prefixes but cannot resolve a tracked variable; the variable resolver handles only bare/eval references and cannot skip prefixes.
 - Round-5 plan: add 10 permanent RED forms (quoted/unquoted `exec`, `command`, `nohup`, and `env` with assignment, each multiline and same-line), then unify shell command-position resolution so literal and tracked-variable terminal tokens traverse the same prefix parser. Retain an independent variable-reference backstop, the 14/14 inventory, and every round-4 regression.
+- Mos stopping-criterion augment: command parsing is explicitly best-effort rather than a complete shell interpreter. Add B1 proving a parser-exotic alias launch with the raw dangerous flag is still RED by primitive anchoring, and B2 proving a parser-missed non-dangerous alias launch reaches the global `.*` hook and fails closed with `GATE_UNAVAILABLE` when no lease session exists. Document A (realistic parser matrix) + B (robust residual backstops); fresh reviewers supply criterion C (no new non-overlapping finding).
+- RED commit `91a4a983`: all 10 prefix×variable cases failed as expected before the fix—quoted/unquoted `exec`, `command`, `nohup`, and `env A=1`, each in multiline and same-line assignment shapes.
+- GREEN structural resolution: `shell_command_tokens()` now owns command-position prefix skipping for both literal and variable callers, including nested `exec`/`command`/`nohup`/`env` ordering. `runtime_variables` is threaded into `is_shell_direct_invocation()` and the same terminal-token resolver backs `executes_runtime_variable()`; exact `$v` and `${v}` references are resolved after `shlex` removes quoting. Same-line runtime assignment delimiters include shell operators.
+- Residual backstops: B1 proves alias-indirected dangerous mode is classified `dangerous-primitive` even though the parser does not resolve the alias. B2 proves a non-dangerous alias residual remains parser-missed, then verifies the shipped global `.*` Claude hook and status-2 `GATE_UNAVAILABLE` denial for representative read, mutator, and custom/MCP tools without a lease session.
+- Stopping-criterion evidence A+B is committed in tests and architecture docs; C remains the fresh exact-head terra/Opus determination. Repository inventory remains exactly **14 gated / 14 total**.
+- Fresh round-5 coverage: permanent guard remains **97%** branch-aware aggregate (251 statements, 112 branches). Locked-good launcher and mutator-gate executables remain unchanged at their round-4 **100% / 100%** evidence.
+- Fresh round-5 gates: real-socket acceptance **50/50**; persistence **10/10**; launcher/gate **14/14**; guard **12/12**; coord **19/19**; Mosaic **1385/1385**; root **43/43**; typecheck **42/42**; lint **23/23**; format and diff checks green.