feat(policy): mission-level capability policy - least-privilege intersection (#30)

- mission schema: optional capabilities.tools (same validation as task)
- merge semantics in runTask: neither -> none; mission only -> mission;
  task only -> task; both -> intersection (task narrows, never widens);
  empty intersection -> tool-free run with an explicit stderr note
- result.json records EFFECTIVE tools; task/mission snapshots remain the
  immutable declaration of intent
- adapters unchanged; host-side only (no image change, 0.0.6 still active)
- task suite +5 cases (41 total): all four merge cases asserted from run
  evidence + invalid mission capabilities rejected

Policy decision recorded: missions govern; tasks cannot escalate.

Closes #30

scripts/test-task.sh

@@ -157,6 +157,35 @@ EOF
         || check "mission section present after retry (relative path resolved)" 1
     expect_exit "retry of missing run exits 4" 4 -- \
         env MOSAIC_CONFIG="$SANDBOX/mock-adapters.json" node scripts/mosaic-task.mjs retry r-missing
+
+    # capability policy (M9): least-privilege intersection
+    POL="$SANDBOX/data/workspaces"; mkdir -p "$POL"
+    pol_run() { # missionTools(ABSENT|json) taskTools(ABSENT|json) -> stderr MOSAIC_TOOLS value
+        if [ "$1" = "ABSENT" ]; then
+            printf '{"missionVersion":1,"id":"m-pol","objective":"o"}' > "$SANDBOX/pol-m.json"
+        else
+            printf '{"missionVersion":1,"id":"m-pol","objective":"o","capabilities":{"tools":[%s]}}' "$1" > "$SANDBOX/pol-m.json"
+        fi
+        if [ "$2" = "ABSENT" ]; then
+            printf '{"taskVersion":1,"id":"t-pol","prompt":"x","mission":"pol-m.json"}' > "$SANDBOX/pol-t.json"
+        else
+            printf '{"taskVersion":1,"id":"t-pol","prompt":"x","mission":"pol-m.json","capabilities":{"tools":[%s]}}' "$2" > "$SANDBOX/pol-t.json"
+        fi
+        env MOSAIC_CONFIG="$SANDBOX/mock-adapters.json" MOSAIC_MOCK_RESPONSE=MOCKED \
+            scripts/run-task.sh run "$SANDBOX/pol-t.json" >/dev/null 2>&1
+        grep -o '^MOSAIC_TOOLS=.*' "$(ls -dt "$SANDBOX/data/runs"/r-* | head -1)/stderr.txt" 2>/dev/null
+    }
+    [ "$(pol_run '"read","bash"' ABSENT)" = 'MOSAIC_TOOLS=read,bash' ] \
+        && check "policy: mission only -> mission tools" 0 || check "policy: mission only -> mission tools" 1
+    [ "$(pol_run ABSENT '"read","bash"')" = 'MOSAIC_TOOLS=read,bash' ] \
+        && check "policy: task only -> task tools" 0 || check "policy: task only -> task tools" 1
+    [ "$(pol_run '"read"' '"bash","read"')" = 'MOSAIC_TOOLS=read' ] \
+        && check "policy: both -> intersection (task narrowed)" 0 || check "policy: both -> intersection (task narrowed)" 1
+    [ "$(pol_run '"grep"' '"bash","read"')" = 'MOSAIC_TOOLS=' ] \
+        && check "policy: empty intersection -> tool-free" 0 || check "policy: empty intersection -> tool-free" 1
+    printf '{"missionVersion":1,"id":"m-pol","objective":"o","capabilities":{"tools":["sudo"]}}' > "$SANDBOX/pol-m.json"
+    expect_exit "invalid mission capabilities rejected" 2 -- \
+        env MOSAIC_CONFIG="$SANDBOX/mock-adapters.json" $TASK validate "$SANDBOX/pol-t.json"
 else
     echo "skip adapter seam cases (docker daemon unavailable)"
 fi