ci: drop check-env-mutation.sh, rely on clippy disallowed_methods

clippy.toml already bans std::env::{set_var,remove_var} via
disallowed_methods, and every existing call site carries a scoped
#[expect(clippy::disallowed_methods, reason = "...")]. The shell grep
is redundant and forced a second, less granular allowlist.

Also update server-secrets-strategy.md to describe clippy as the
enforcement mechanism.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/rust.yml

@@ -47,7 +47,6 @@ jobs:
         with:
           persist-credentials: false
       - run: bin/dev/check-boundary.sh
-      - run: bin/dev/check-env-mutation.sh
 
   fmt:
     name: Format

bin/dev/check-env-mutation.sh

@@ -1,44 +0,0 @@
-#!/usr/bin/env bash
-set -euo pipefail
-
-cd "$(dirname "$0")/../.."
-
-if command -v rg >/dev/null 2>&1; then
-  matches=$(rg -n 'std::env::(set_var|remove_var)' --glob '*.rs' || true)
-else
-  matches=$(grep -R -n -E 'std::env::(set_var|remove_var)' . --include='*.rs' --exclude-dir=target --exclude-dir=.git || true)
-fi
-
-fail=0
-while IFS= read -r match; do
-  [[ -z "$match" ]] && continue
-
-  path=${match%%:*}
-  rest=${match#*:}
-  line=${rest#*:}
-  line=${line#"${line%%[![:space:]]*}"}
-
-  case "$path:$line" in
-    "lib/crates/fabro-telemetry/src/spawn.rs:std::env::set_var(key, value);" | \
-    "lib/crates/fabro-telemetry/src/spawn.rs:std::env::remove_var(key);" | \
-    'lib/crates/fabro-cli/src/main.rs:std::env::remove_var("FABRO_WORKER_TOKEN");' | \
-    'lib/crates/fabro-server/src/install.rs:std::env::set_var("FABRO_TEST_IN_MEMORY_STORE", "1");')
-      continue
-      ;;
-  esac
-
-  echo "process env mutation check failed: $match" >&2
-  fail=1
-done <<< "$matches"
-
-if [[ $fail -ne 0 ]]; then
-  cat >&2 <<'EOF'
-
-Do not mutate process-wide env with std::env::set_var/remove_var.
-Inject env at construction time or on child-process Command values instead.
-See docs-internal/server-secrets-strategy.md.
-EOF
-  exit 1
-fi
-
-echo "Process env mutation checks passed."

docs-internal/server-secrets-strategy.md

@@ -9,7 +9,7 @@ This document defines how Fabro handles server-level secrets.
 - Resolution is snapshot-based: env and file are read once at construction, then treated as immutable for the life of the process.
 - `process env` wins over `server.env` on conflicts.
 - `fabro server start` never generates secrets. Missing required secrets are a startup error.
-- `std::env::set_var` and `std::env::remove_var` are banned workspace-wide. Tests are not exempt. CI enforces this with `bin/dev/check-env-mutation.sh` so broad clippy suppressions cannot bypass it.
+- `std::env::set_var` and `std::env::remove_var` are banned workspace-wide. Tests are not exempt. Enforced by clippy via `disallowed_methods` in `clippy.toml`; intentional exceptions must be annotated with a scoped `#[expect(clippy::disallowed_methods, reason = "...")]` at the call site.
 
 ## Active Server Secrets